Hi Lithium,
You can use IDM to achieve your objective. You can put the common signature .jpeg in the set and then use it for exclusions. You can't use IDM for exclusions on Endpoint but you can use it for detection. This is from the Admin Guide:
You use Indexed Document Matching (IDM) to protect confidential, proprietary,
or sensitive content and information stored in documents and files. For example,
you can use IDM to protect financial report data stored in Microsoft Office
documents, or merger and acquisition information stored in PDF files, or source
code stored in text-based files. You can also use IDM to protect binary files, such
as JPEG images, CAD designs, and multimedia files. In addition, you can use IDM
to detect partial or derived content, such as text that has been copy/pasted from
one document to another.
Best,
Ryan