Alright, just be warned it's a fairly complex process. It's also rather useless, but it was a good coding exercise nonetheless.
1. Download and install IrFanView. You can also just download the portable installer-free version if you'd like. All you really need is the i_view32.exe file.
2. Download the v3.0 package from
http://danthemanms2.googlepages.com/multibootiv.html and unzip it.
3. Double-click the createFolders.bat file to create the folders you need.
4. Double-click convert.jar to open the conversion program.
5. Set the paths to i_view32.exe and the "MultibootImageViewer3" folder (only needs to be done once, as it saves the paths you select)
6. Browse for the images to convert. You can select multiple files.
7. Click the button to save the image_convert.bat file.
8. Double-click the image_convert.bat file that was just created to actually convert the images.
At this point, the converted images will be in their respective folders (240x64 etc). Run the respective *.bat file for the resolution you want, or run the "runAll.bat" file to do all of them. Click OK for each image (I can't get rid of this unfortunately).
Then go to the outMB folder to see your files. Copy to your card and run. Hopefully they'll work on the GBAMP, though of course there's the chance that they won't.
If you want to then clean out all the converted images so it doesn't rebuild them into *.gba files whenever you run the runAll.bat file, simply delete all the folders except DragonBASIC, outGBA, outMB, and SRC, and then run the creatFolders.bat file again.