Great news! I finally solved the distorted back sprite problem I think...
Here's what I did.
1) I followed Jagold's tutorial when I made my custom back sprite. After decreasing the color depth from 256 to 16, copy the exact RGB values of your front sprite, I mean the palette, to your back sprite.
2) You must copy all 16 color of your front sprite palette into your back sprite. After that use Unlz-GBA and first insert your front sprite and then your back sprite. Jagold's Dark Lugia looks AWESOME now!!
I want to post some screenies but I'll have to wait then. Thanks for helping me out Jagold and all the members of this community