What do you mean by "take anything up to 20 minutes to go through my Norton scanner."?
Is this the time it takes to send the mail or to scan the mail before sending it. If the former then you could try reducing the size of the pictures before sending them, if the problem is with the latter, then try re-installing norton or better yet get a different Anti-Virus package, try the free AVG click here or my prefered software Nod32 click here it costs £23 to buy & £15 a year for updates but it's never failed my yet and uses loads less resources than Norton
All my outgoing e-mail, with or without attachments, are scanned by Norton antivirus.
I often send very large jpg attachments, (to friends, who like me, have broadband), and the actual Norton scanning process takes only a few seconds. The transmission takes longer of course. What file sizes are you sending?.
Are you on broadband?; if not, the bottleneck is more likely to be the speed limitation of dial-up.