We've had a similar thing here, although our problem occurs with Office 365's version of Outlook. We've only seen it on background images, but I'm going to test it with standard image tags too.
Hopefully someone here knows what's going on!
Believe me, that would be our first choice! Unfortunately, the client is adamant about having the PDFs as attachments (something about them being more immediate *rolls eyes*). We've sent it out as is now, but I still find it pretty strange that it is happening, especially with the fact that it's the number of attachments that causes it, not the attachments themselves.