Thanks for the advice, guys. Guess I have to work a bit more with GIMP's editing tools.
And cheers for the compliment, Arseen!

I'll see if I can find a better seal when I finish with the front.
Also, I spent today putting Puzznic NTSC together:

Shouldn't need too much touching up since I redid much of it from scratch. The back was fairly easy but the front was a pain in the ass, lol. Took me a lot longer than I was expecting but I think it turned out well.
Edit: Something looked a bit 'off' on the cover; the square on the right side was too long compared to the rest of the squares on the front (I didn't stretch this cover so I drew in the extra on each side) so I shaved the edge off and made it match the size of the other shapes.