Use only the spritesheet

Currently, all pokemon images are separated, whereas I could try to extract directly the correct sprite, use it and remove it afterwards.

The issue names no files, tests, or entry points. Start by locating where Pokémon images are loaded and rendered, then inspect how the spritesheet is used or could be accessed. Done means the correct sprite is extracted from the spritesheet without retaining separate image files, with the existing image behavior preserved.

Written by the indexing model from the issue text.