I am not having any problems so post the error in the bug report thread. I just took a battle screenshot, converted it to jpg and put it in, and it worked. here is my test flag:
Naming convention doesn't seem to matter, but file format might. Are you sure these are jpegs? perhaps opening it in paint and saving as jpg will work.
How do I add additional flags? When I download a flag as .jpg, follow the naming convention of flag***.jpg, and place it in the /flags directory I cannot select it from the Races screen. Instead, I receive an error.