This could be for a variety of reasons. If you are using Outlook as an email client then there is likely an issue there (I consider Outlook 2007 a personal enemy). I encountered a similar issue due to my large format display. This article best describes the issue I was having and the fix:
I suggest testing the email in different email applications as well as other computers to see if the problem persists. If so, it is possible that the image is being sized by an inline style string.
I am sure there are some more experts here that can weigh in. I suggest donuts to cope in the meantime.
You will want to make sure that you are using a high enough resolution image. Also, don't fool around with resizing images via HTML or inline CSS. I would just make sure they are the correct size for the space you are putting them in.
Hope this helps!