Can you include your photography work into your showreel or do you have to create a separate showreel dedicated to photography? I personally rather have both my video and photography work to be in the same video but I personally have not seen a showreel like that before.
Still photos thrown in with moving images will take your audience out of the action and will hurt the overall impact in most cases. There are a few situations where it could work, but it depends on how well you edit it. Part of your value as a content creator is your ability to tell a story and get a mood out of your audience; use those skills to determine if stills are a good idea or not. Or just try it. There are no hard rules, just what sells and what doesn't.