ferran said:
I am currently developing a game engine but I'm not quite sure where to find good assets for the graphics
To be clear, you wrote about building an ENGINE and not about making a SINGLE GAME.
An engine needs to be able to handle a variety of graphics assets from a variety of sources. If you're building an engine you need to be sure your code can handle all the tiny little implementation variations that come from all the different sources, tiny differences between Photoshop, Gimp, Substance Painter, Substance Designer, ZBrush, MudBox, and countless others. You'll need assets from as many varied sources as you can find and you'll need to be sure your tools work with all of them.
If you're building a single game, then it's about whatever assets you can find, and as others wrote you build your game around whatever you can cobble together. It can be done, and is commonly done at game jams and other short-term projects.