I will post a "home made" rule later that reduce Image Spam at least a 50% (works at antivirus.exim). You know 50% of the image spam is attached as a gif file and the other 50% is embedded into the message.
The rule may be kinda rude cause it deletes all email received to the server that includes .gif attachments. So if a legitimate user is sending for example a gif banner or something to one of the customers in the server as an attachment, it´ll be deleted. (or you can make a white list to avoid some address from being deleted

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The only thing you can to is to tell your customers (to avoid complaints), is to explain them the image spam situation nowadays and have the gif files compressed with zip or rar. (you can do this meanwhile until a good solution to this be done)
The rule doesnt affect users from the server sending out gif files. This ones can be sent without any problem.
The rule is applied to esmtpb, no to the esmtpa (authenticated one).
With this you will only receive those image spam with embedded gif in them. Gif file attachment will be deleted and logged into a file for further research.
Also i have other rule that filters by content-type, for example korean iso, 7-bit, etc will be blocked if you want.
Sometime in the near future spammers change their methods and use jpg instead of gif an so on with other extensions. We have to wait spamassassin next release (I suppose 3.1.8) if it has some solution to this.
This is a war that never ends.
Regards