Quick Dial will sync as part of a normal Firefox sync. You don’t have to create an account at somewebsite.com the provenance of which you know nothing.
It allows you to choose a background image from your own image library, but the backgrounds will not sync; they must be selected locally on each machine.
Quick Dial also gives you more options for configuring the appearance of the individual dials than I have encountered with any other speed dial plugin. For example, you can change the labels of the individual dials, as well as the color of their font and background. This is useful when the default title is “All the News You Can Use | Some City’s Newspaper”; that can be edited down to “Some City’s Newspaper.”
I like it so much that I kicked in a small donation to the maintainer and you can too.