What version of Fire Works? this sounds like an issue with some versions of Fire Works not being able to load from the theme it edit button.
Instead, you will need to open fire works and manually find the image file you wish to update.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.