I tried to upload an image as a new sprite(The picture was a saved image of the stage on a previous project) but it wouldn't work. What kind of picture do I need to put into Scratch?

If you want to upload a sprite, you have to upload a .sprite2 file. Create a sprite, go to costumes, then click on New Costume.

I think the problem is that you can't upload a stage as a sprite (possibly, I don't know).

Is it .png? (If it is it should work.) But try duplicating the stage if it is .png.