This is how the image browser looks in the GUI of Akiko:
The idea behind the browser is quite simple. To make Akiko display your favorite image when you click on a game, put the image file into the Images\CD32 folder for the CD32 games, Images\CDTV for the CDTV games and Images\Other for the other emulation options. The filename must be the name of the game then plus either .bmp for bitmap files or .jpg for jpeg files.
For example: You would like Akiko to display the file My Cool Pic.jpg when you click on Soccer Kid. So you just copy the file My Cool Pic.jpg to Images\CD32\Soccer Kid.jpg in the Akiko root folder.
Be sure that the image is either in jpeg or in bitmap format! You can also put a file named default.bmp or default.jpg in those folders. These images will be displayed then if no picture is found for the current game. The image will be scaled to fit into the image box in the GUI but proportions will be respected.
The image browser is primilarily meant to display the covers of the current game but of course you can use all pictures you like.