There is now a graphical interface online for Retropixels, the tool I made for converting images to Commodore 64 format. For now it supports multicolor and hires bitmaps, so no sprites or FLI/AFLI. For that you still need the command line tool.
A new feature in this version is the ability to disable certain colors from the palette. This will make it to the command line version aswell, eventually.
The web version is 100% javascript running in your browser, there are no server-side components, so the images you convert never leave your computer.