Excellent add-on that has recently hit a couple of speed-bumps... two things have happened.
1) The Host File Server has changed from: http://imagehost-grabber.com/hostf.xml to: http://imagehost-grabber.eu/hostf.xml
2) With the automatic signing of AMO hosted add-ons (the .1-signed thing) the add-on was, in effect, reset to some really old version of hostf.xml
So, defunct Host File Server address and an old hostf.xml file. The solution is to manually change the Host File Server to the newer address. Under the ImageHost Grabber menu pick: 'Host File Editor' and click the: 'Modify hostf servers' button.
Use: 'Add Host File Server' to add: http://imagehost-grabber.eu/hostf.xml
Use: 'Delete Host File Server' to remove: http://imagehost-grabber.com/hostf.xml
Click: 'Update File' to confirm the changes.
Exit the Host File Servers panel.
Finally, click: 'Update from online Host File' to get the latest hostf.xml file.
There is some discussion here: http://imagehost-grabber.eu/index.php?topic=3217.0
There is a third thing. As these files are held within the add-on itself, making these changes 'breaks' the signing. So, at some point in the future the add-on will be flagged as being invalid. Only the add-on author will be able to fix this (and, indeed, will need to fix it for the add-on to work wherever it is hosted). The active versions of the two files: hostf_servers.xml, and hostf.xml, really need to be held inside the profile folder but outside of the add-on's extension folder.
There is some initial discussion of this here: http://imagehost-grabber.eu/index.php?topic=3099.0