This seems to be a poor way to approach email for a few reasons:
1- Many people don't have images enabled. Even if I'm not worried about the safety of the images from the Hillary campaign, it's much easier for me to go onto my next piece of mail than it is to enable them.
2- As long as they're using images, they may as well actually use them well! Although they say that a picture is worth a thousand words, a picture of words is worth no more - and in this case, less - than the words themselves. It seems that the Hillary campaign is using images instead of text for some shortcut - whose benefit is still unknown to me - rather than fully utilizing the strength of the medium.
3- You can share data more easily when it is in text form than when it is an image. This is especially important given the proliferation of such quick and informal information-disseminating venues such as blogs, Twitter and myriad instant messaging platforms. Even if you have images enabled, someone has to take the time to type out whatever information they want to share using those tools. With a text-based message, you can simply copy and paste. Simply put, information in text form has better legs than information in image form - particularly appealing/viral pictures aside.