Jasper Vijn has released a new image converterfor the GBA(ds too), heres the info:

Yay, yet another image converter. Do we really need another one? Well maybe, maybe not. As far as I can tell there are two very complete gfx exporters right now: gfx2gba (by Markus, not
the other one) and cog (by OneManBand). The second one seems not to be available anymore, and the first one wasn't quite able to do what
I wanted. Also, both of these are pure command-line tools. Wingit is a GUI tool, making it easier to get started. The main view offers a preview of the image you want to export, which is always nice.
There's a palette viewer too. By using FreeImage, it can read pretty much every kind of image (yes, GIFs too). As for the exporter itself, I've pretty much crammed it full of options. There are separate sections for palette, image and map control, which include cropping the exported image, various bitdepths (including 1 and 2bpp for use BitUnpack) compression and tile-set reduction. There are also options for tile-grouping, not only can this be beneficial for sprites in 1d mode, but you can also
use it to create meta-maps of arbitrary sizes. The output file(s) can be C, GNU asm or binary, a header file is optional and there are choices between u8, u16, and u32 type-sizes. Naturally, the variable name can be altered independently from the file-name.

While the program is useful as is, there is always room for more. There are a few other things I'd like to add, like vertical map layout, batching options, merging palettes and tilesets, and an extra
output format or two. And, of course, proper documentation. But I'd also like to get back to real GBA stuff so I'm leaving it as is for now.
Btw, if you would like to see things like this added, the source code is freely available, so why not try it yourself? (Then again, seeing my code...)

Download Here