I am using ZD with a google apps account using IMAP. I was as frustrated as you guys but have managed to solve this for my setup without destroying my account. I edited the zimbra-attrs.xml and initially only changed the default value from 10240 to 20480 then tried my longer html signature and got the same error even after quitting and re-starting ZD. Then I theorised that in fact there was no 'value=' argument in the attr line even though there was one in the previous attribute so I added value="0" saved the file then re-started ZD and tried again but still got the same error. However, I then changed the format of my signature from HTML back to plain text then changed it back to html again, re-added my image reference and saved the longer html signature without problem!
Not sure whether it was the adding of the value argument or making the client re-check the attrs file by changing from html to text to html but it worked for me.
Hope this helps some of you.