Have no fear! Making game icons is easy!
I use a utility called PictIcon
found on aminet.
The Icon in the bottom left of the picture allows any IFF, GIF etc to be dragged and dropped onto it from which it creates a neat icon!
To get the picture for the game, just run it in WinUAE and use the key combo "alt printscreen" to save the title screen (or whatever you want) to the clipboard. Then use your favourite paint package to save it in a compatible format!