My sister-in-law used to use a free service on her Bellsouth email account, one that attaches cute graphics to the bottom of the email. Other folks I know have professional logo gifs or signature gifs embedded in their outgoing email. Some of my friends send me emails that have tiny little virtual business cards attached, "vcf files", the idea being that these files can be easily dragged and dropped into MS Outlook. And several of my friends have email that attaches an odd little file called "winmail dat" to the bottom of their emails.
All of these are considered unsolicited attachments when they show up in an agent's email inbox; nine times out of ten, agents (including myself) receiving that email will delete it unread. The alternative is opening an attachment from a complete stranger and possibly unleashing a computer virus in your hard drive. This is the reason that so many agents who accept email queries also ask that you send no unsolicited attachments.
Do yourself a favor and double check that your outgoing email doesn't carry any unwanted attachments of any kind. That cute little dancing puppy animated gif could be costing you your query letter.