Don’t think a front-end woud be much use for ImageMagick… A GUI to cover all it’s features would be a leetle unwieldy! For the ‘convert’ command, for instance, I counted 167 options (each with its own variant parameters)!
I’m a great believer in GUI/Command-line complementarity. Each has its own advantages, neither to the exclusion of the other. For the stuff you do often, and much in the same way, the GUI is supreme, but the command-line gives a flexibility the GUI can’t. Which doesn’t mean it has to be complex or tiresome. The command I used to reduce colour was simply:
convert tiger.png -colors 16 tiger16.png
Actually, I often combine the two realms – with xicon 
A command line I use a lot goes into a xicon script, so that I can just drag and drop its subject on that. And with something like Gadgeteer, you can even provide for a few variable parameters. But not 167 of them!