Update: I figured out that this only happens when I paste images into a table. The problem began when they upgraded us to Microsoft Office 2013, so you probably can't recreate it in earlier versions. I created another posting outlining the details.

Thank you for figuring this out! Only when pasting into tables.. very odd

Yeah, sometimes objects in tables have their own set of rules. Several years ago I had an application that pulled info from an Access database and entered it into a Word table. Worked great, but spellcheck didn't work on the pasted text. Worked for stuff manually entered, but not the stuff that was inserted via mail merge. weird.