hey and thanks for asking! unfortunately i'm not good at both gba development and explaining things at all, so i can only advise to check out the gbadev forum or discord server's #graphics channel and ask around there, or see if the getting started guide on gbadev.net has some useful info. i will, however, say that displaying full color images on gba screen does require a decent amount of programming sorcery and, depending on the engine/gba display mode of your choice, reducing palette color count (and possibly tile count) before importing is the general way to go for graphical assets.
best of luck with your gba vn, looking forward to see what you're working on!