Not sure why you're getting that error. I also own AJE and just tried editing a region (drew a big red heart on Londinium), saved it as a bmp and the game loaded fine.
I used Photoshop Elements. I just opened the file, edited it, then hit Save. A dialogue popped up and I selected BMP, then another dialogue opened. I selected 32 bit there (not sure that's necessary, though) and hit Save.
Be sure you didn't accidentally change the name of the file. Or change its size (when you added your graphics, did you somehow change the dimensions of the file?).
If your file has the same name as the original and is saved as a BMP and is of the same dimensions, it should load fine - unless there is something about the program you're using that works differently than Photoshop.