In trying to find info on image file size best practices, I found a few things.
ExactTarget will not host an image file over 200KB. That’s not an issue for us as we use a CDN, but something to keep in mind.
Campaign Monitor recommends a 1MB max.
Email on Acid says that in their test they found that “..image file size did not have any impact on deliverability”, but they tested a max size of only 696KB.
MailChimp recommends keeping file sizes less than 1MB. Larger files load slowly in your subscribers’ inboxes.
I heard of a study done that found that 1.2MB was the cutoff, as any larger resulted in complaints, which in turn resulted in deliverability issues. Of course there are other issues with large images when it comes to load time, but I wanted to know if that could result in deliverability issues, and apparently it can.