I discovered, the hard way, that the Android Gmail client, using naked HTML (with a default naked DTD specification), does exhibit this problem. It only responds to <img /> [i.e., not <image />]. gmail.com is fine with <image />, but not the Android gmail client.
It does indeed have <image> tags as part of the spec, in order to account for about 0.2% of websites per a study done in December 2005. See this comment for the link in the spec: https://news.ycombinator.com/item?id=16462230