Another potential method (or an additional filter) for finding posts with photos, or more specifically posts whose main purpose is a photo or image is to use use the post type discovery algorithm. Given that Micro.blog is built on a variety of IndieWeb building blocks, most photos could/should have a class of u-photo on their img tags, so you could search for these instead or in addition to. I believe there are a set of parsers and tools out there that do this in a few languages already and someone in the IndieWeb Dev chat can direct you to them if they’re not linked to the page above.