I have individual animation frames for a 2D game, each as a separate PNG file. I need to convert them into a single tilesheet/framesheet (not sure if there is a recognized term) which arranges these in rows/columns. It gets really boring doing it by hand using copy-paste so is there a simple tool out there which can help me?

I don't use Photoshop or GIMP, my only image program is Paint.NET since I'm just a programmer

I don't have a recommendation because I'm also just a programmer ;) and never used one of those tools, but googling for "sprite sheet maker", "... generator", "... creator" brings up many applications that can do something like that, e.g. GlueIt, SpritePacker (seems Mac OS only), Predator (a commercial one, although with 15$ relatively inexpensive), Sprite Sheet Stitcher, Texture Atlas Generator, ...

I'd recommend glueit

Awesome, glueit is simple but exactly what I was looking for.