I might be able to help on this one. There are a couple of pieces to get pictures on a plug to show up right.
1) If your game is named "thisgame.7z" then the cover should be named "thisgame.7z.png". This is different than wii covers that do not need the extension as part of the name.
2) In the plugin file, make sure that the path is right. So, as an example, if your covers are in SD:\wiiflow\boxcovers\atari72\ then in the plugin ini file for atari72 you would want their to be this line: coverfolder=atari72
3) In the same plugin file like the other guys mentioned. You have to have the filetype listed, so you would have a line like this: filetypes=.smc|.fig|.sfc|.swc|.zip|.7z (this example is from my Super Nintendo ini, since I don't play atari72).
4) Lastly make sure the game with no cover can be seen in wiiflow and run "reload cache" off the settings menu. This will make wiiflow verify that everything that has covers will have them shown.
So, to my knowledge those are the keys.