I suppose it depends on what the content is. If you're trying to convey an emotional appeal, an image can be very compelling. I could see using images to complement the text in that case. If you don't see images, you can read the copy. If you do see them, you get both the message and the supporting visual.
In the case of Hillary's message above, it was truly baffling that they would use an image since the image (once you loaded it) was an image of... wait for it... COPY. They had created an image that was nothing but words. And it wasn't even terribly well formatted copy. There was no reason not to send it as text.
What's worse, is the text contains two links, but the second (the link to join the event committee) would not work if you clicked on it.
This is the loaded image (click to enlarge):

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Question:
It doesn't seem to me as if using images rather than text offers any benefit other than the fact that the resulting message will look prettier. That assumes, of course, that images are enabled.
Are there any benefits that I'm overlooking? Is it easier to create these image emails than a text blast?
Also, in light of Google's recent update to its search algorithm to reflect the speed at which a site loads, it seems like campaigns - and pretty much anyone attempting to influence people - would be more attuned to matters of limited bandwidth and the reduced speed at which the image-laden emails load in comparison with their text counterparts. Or perhaps the difference in access times are negligible.
Like I said in the subject line: I had a couple of questions...
More about Hillary's email strategy here: http://tinyurl.com/yqa7bw