The GBA loads the necessary tile data into the I/O (for sprites) and VRAM (backgrounds).  Using VBA, I can locate the tiles loader, find the data in a hex editor and change the data to load the necessary graphics. 

But I've no idea on how to find the x and y values which tell the game where about the image has to be displayed on the screen.  How is it possible to locate them?  I've had no luck finding any documents about how the GBA does this besides one which I misplaced. >_<

Anyone able to help?   

Homebrew coding sites are absolutely useless for romhacking in my experience Especially when it comes to GBA. The connected community is also very hostile to ours to boot, never ever give away you're romhacking when you ask them a question about one of their tools (even if the question itself would have absolutely no connection with romhacking), or they'll refuse to help you.

RadioShadow: You'll usually have to debug and change the ASM code that loads the tilemap. Unless the game just loads a tilemap for the whole screen at once, but that's not so common. It sounds though as if you already found the routine which sends the tilemap data to VRAM. Well, just change the VRAM destination address then.

what exactly are you trying to edit? background related stuff or sprite related?