What you need to make a texture mod / retexture / "Repaint" of this game are the following:

• Insaniquarium! deluxe
  
• Photo editing software(preferably photoshop, but MS paint works)
  
• Knowledge of where the game files are

and you are ready!

that's simple,

1 > Go to your steam folder
2 > open "steamapps"
3 > go to "common"
4 > Open the file named "Insaniquarium! deluxe"
5 > The file you want is named "Images"

Your directory (thing at top) should look like this, except the "Steem" should be the folder you have your steam in

well, first i reccomend you COPY AND PASTE THE "images" FOLDER SOMEWHERE SAFE! just in case you want to revert and or recover a texture

there are two texture types:

• Bodies
  
• Sprites

the "bodies" tell the game where to cut out the sprites like a stencil


You'll want to make sure that everything will match the outline or it won't be seen

you can try to extend the outline yourself but it is not recommended,
try to stick to the normal bodies if you can, because editing them could cause hit box issues

much like poor ol' frank here:

so yeah, not much more, just draw on it, just be warned that the solid color area isn't seen

so you either ♥♥♥♥♥♥ up, uninstalled the game, or a friend sent you a texture pack of theirs,
how do you install it?

• FIRST!
  you save the folder and rename it to "images" (if it isn't already) so that it has the same folder name
  
• THEN!
  you'll want to click and drag the folder with the new textures into the game files and when prompted, hit "replace"(Note: it will only work if you replace the file)
  
• NEXT!
  wait for all 400+ files to transfer...
  
• FINALLY!
  enjoy the pain!