Has anybody gone about writing an alternate image gallery for fck?

Is is really easy. You don't need to write a new provider. Just create a new .aspx that handle the images the way you want (You can base your code in the current .aspx)

After that, you only need to change the Gallery path in the FCKeditor section at web.config

Just a quick question, does this method allow me to change out the underlying store for the files? As we wanted our documents to be stored in a document management system we use.