I am wondering how and to what extent free art / sprites / 3d models can be used in an own game.

For example, lets say in the case of an animated mickey-mouse sprite, where the readme inside states:

"This sprite may be freely distributed UNALTERED. Which means, you can't pull the readme file out of the zip,or add your own stuff to it and pass it along as your own!"

I believe including it in the game an selling it bundled will not be allowed.

On the other hand, if the user downloads the free model himself and installs it in the game-folder, there wont be any problem.

Now where is the boundary ?

Lets consider the following cases:

Like what happens if the game does not contain this sprite and

-downloads and installs it automatically after installation ?

-downloads and installs it on request (pushing a button) ?

-contains a script to download and install the sprite ?

Terms of the licence should be clear. When in doubt, contact them and clarify the license.

I would interpret that statement as NOT allowing it in a game. I would not use any of your side-channel methods.

They meant for the entire package to be unaltered. It is a standalone package. By putting it into a game you are using it outside the distributed package. Even if you have it as a side download, you are still opening up the package and using the parts. Your game will likely have all your branding and your credit info and money going to you; none of it goes to the package creator. The license prohibited both things.

In your case you mentioned "mickey-mouse". If you mean the Disney character, you are almost certainly not properly licensed from Disney if you are asking. Disney (like all major companies) takes active steps to protect all of their valuable property, and Mickey Mouse is top of that list.

Is there any contact info in the readme? I would contact the person if possible to simply ask permission. Worst case is they say no.

I think it's likely from what they wrote that they were simply worried about someone taking their art and claiming it as their own (shoving it into their own art pack), rather than someone actually using it as intended as a game sprite (or they wouldn't have released it in such a format). This is coming from someone who has slapped together sprite packs and handed them out like candy. I simply ask for a credit somewhere, and I'm afraid I didn't bend over backwards researching the licenses when I made mine so I can imagine there might be a gap between their intention and strictest interpretation of their wording.

Of course, their intention may have also been to release the resources for use in non-commercial games only, so I would really just see if you could their permission directly.

And if it really is Micky Mouse, you have a bigger problem to worry about, as Frob pointed out.

I think it's likely from what they wrote that they were simply worried about someone taking their art and claiming it as their own (shoving it into their own art pack), rather than someone actually using it as intended as a game sprite (or they wouldn't have released it in such a format). This is coming from someone who has slapped together sprite packs and handed them out like candy. I simply ask for a credit somewhere, and I'm afraid I didn't bend over backwards researching the licenses when I made mine so I can imagine there might be a gap between their intention and strictest interpretation of their wording.

I think the point in Frobs comment was that you should not assume anything, and if the license is unclear, you should assume 'no' until you clear thing up with the author. While you may be, and probably is right in your assumption, it is the very act of assuming that it 'dangerous'. It doesn't say anything about usage at all?

If it is Mickey Mouse, you can be sure you can't use the sprite no matter what permissions the author gave. Disney pushes the envelope all the time for how old a work can be before it can be considered 'public domain'. According to initial intentions, the oldest animated shorts, should have been in public domain already, but I figure 'money talks'. Regardless: That Disney is lobbying for law changes working against public, in favour of economic interests shows just how fiercely they will protect their intellectual property. Apart from that, Mickey Mouse would probably not be in public domain, even though the oldest shorts would, since Disney is still actively using and utilising that figure in present day.

Ok, so in case of

- a sprite that is not any famous character like mickey mouse, its sufficient to get the permission of the author. ( I believe then, depending on the case, it can be included directly in the game by giving credits to the author )

- sprites such as mickey mouse, the game could point to a page where the gamer may download and install that file by himself, in case automation of this process is a violation.

now, is hosting a mickey mouse sprite on your own server also against law ? I mean I havent seen Disney taking down deviantart.com for copyright issues of mickey mouse wallpapers e.g.

next, if you game contains an editor where players can create their own character that is then automatically available to all other players via internet, and one player creates a mickey mouse e.g. , what is the situation there ?

is hosting a mickey mouse sprite on your own server also against law ?

It's not that it's "against the law" (no policemen will arrest you for it) - it's a violation of another party's copyright and/or trademark, and you can be sued for it.

if you game contains an editor where players can create their own character that is then automatically available to all other players via internet, and one player creates a mickey mouse e.g. , what is the situation there ?

You could potentially be taken to court by Disney (or whoever owns the character in question if it isn't specifically Mickey Mouse). See for example Marvel v. NCSoft, where the makers of "City of Heroes" were sued because players were able to recreate comic-book characters in game.

A previous thread in which this question came up:
http://www.gamedev.net/topic/653734-legal-risk-in-editors-and-giving-players-freedom-to-edit/

if you game contains an editor where players can create their own character that is then automatically available to all other players via internet, and one player creates a mickey mouse e.g. , what is the situation there ?

You could potentially be taken to court by Disney (or whoever owns the character in question if it isn't specifically Mickey Mouse). See for example Marvel v. NCSoft, where the makers of "City of Heroes" were sued because players were able to recreate comic-book characters in game.

Thank you for that example. I also found how it was resolved:

Marvel and NCsoft did not disclose terms of the settlement, but the result appears to allow NCsoft's games -- both City of Heroes and City of Villains, the expansion released in the interim -- to proceed unhindered, allowing players to create characters as they wish, without physical restrictions on what characters look like.

http://www.gamasutra.com/view/news/98473/NCsoft_Marvel_Settle_City_of_Heroes_Suit.php

No, you did not find out how it was resolved. The key part is "did not disclose terms of the settlement". For all you know NCSoft needed to pay millions of dollars to "proceed unhindered".