FWIW, I and others dislike the percentage width, especially if we narrow the thread window for easier reading of the text. At 40%, narrowing the window makes the type easy to speed read, but makes the picture so small it's barely visible. But if you use a pixel width, the picture dimensions remain stable even if you narrow the window.
Yes, a poster may have to learn a couple of widely-applicable pixel widths or heights, but they'd be doing a favor to us visual learners who glean detailed info from photos.
Islander7, the "HTML Sandbox" link is in the red and blue lines of keywords near the top of the Forum page, so it's always available. You can learn how to post there, and try things out on the Sandbox thread. It's a great help!
Yes, that’s the trade-off on the % parameter. For years I just used a fixed width, but realized that caused a lot of users problems. Too bad there isn’t a way to say “% but not less than X pixels” to limit the shrinkage.