Don't know about Pegasus for sure, but the rest all use Internet Explorer to display messages IIRC. Thunderbird is based on Mozilla.
So, I would suspect more that your problem is spyware related. Or at least tied to something you did, or allowed to happen, to IE.
My email program (Incredimail) stopped recognizing graphics. Loaded Eudora and Pegasus - same problem. Tried Outlook Express - same problem. Finally tried Thunderbird, graphics recognized in this program. Is it posibbly a virus problem that causes this?