One problem is that this is not useful for search engines and other automatic algorithms. This is a serious issue: something like 90% of the traffic on most Stack Exchange sites comes from Google, so a question that can't be Googled is inherently problematic.
This question also presumably can't be indexed very well by Stack Exchange's own servers. For example, it probably won't show up in the "Related" section on the right very often, and it probably won't be suggested as a possible duplicate when someone is asking a question.
All of this could be ameliorated by simply transcribing the text again after the image, possibly with a preface saying "In case you can't read the text above, here's a typed version." It might also work to have the text itself be the alt text for the image, but it's not clear whether Google and Stack Exchange use alt text in their algorithms.
The second problem, of course, is that using this technique detracts from the question itself. This site would be a lot less useful if everyone did this, simply because it would be more work to read the questions. If you try this once and it starts a trend, then that trend might need to be shut down before it got out of hand. This kind of thing may be fun, but it's the sort of fun has the potential to get in the way of the site's main purpose.
So my suggestion would be to use this technique very sparingly, especially now when the site is young and we don't have many questions. I think it would be fantastic to have a few of these next December during the holidays, but for now I think there should be very few questions of this type.