Image is an element for java games/apps that work as grapics. It also use for apps/games title, thumbnail, bar, background, icon etc. It shows a game/app more beautifull.

Type:
There are many type of image (png, jpg, gif, bmp etc.) but in java games/apps mostly use .png type image. We also use .png type of image in our modding apps.

Resolution:
Resolution is most important for image because there are many resolution java phones (128x160 176x220 240x320 etc.) support different resolution games.