When i import my front pokemon sprite with unlz.gba, it goes alright.
But when i import my back pokemon sprite with unlz.gba, when i test it ingame, the palette gets automatically messed up, any helps?
The front sprite uses normal palettes and the backsprite (in unlz.gba) uses the shiny palette. When you import a new sprite, you are changing the palette (even if it's the same image, you're changing around which color goes first in the palette order) so instead of "blue green orange yellow" it becomes "yellow orange green blue"-- all of the blues in the original become yellow, all of the green's become orange, etc. even if the same colors were used.
There are other tools out there that specifically specialize in inserting front and back sprites (Wichu's Advance Sprite Editor, for example) and unlz.gba tends to mess up people when it comes to palettes.