I do it the old fashioned way. I go to the main game graphics folder and backup the original image file - call it roundels_original or whatever. To do this, just click on the name of the file until the text gets highlighted, then change the name. Then, I simply paste the new file in here. It should load after you do a full restart of the game.
NOTE: the game loads the graphics when you first start it, so you need to close the game down completely, add the files you want to use, then start the game up again if you're unsing my old fashioned technique.
Okay, I usually try to avoid the "direct" method, but its working that way. Much better, thanks. Now for those map objective flags!!!