Hi

Thanks for the nice reply! I do not really want to make a long list or something. I think any popular image format accepted by popular engines like Unity, Unreal will do the trick. If you are not sure, I think PNG (for transparency support), JPG (for different compression levels) and may be GIF (just for fun purposes). 

Again, I am not asking for anything in particular, I just thought it would be nice to have those. If you want to stick to only one format, then may be PNG is the best candidate here.

Thanks and good luck