I think I've already looked at those examples, but I may have missed something. Those examples show the use of absolute pixel widths, as in [[Image:screenshot1.jpg|600px]]. I need to be able to use a percent, something like [[Image:screenshot1.jpg|60%]], or something similar. The fixed pixel size means a large image won't expand past the fixed pixel size for larger screens; I want it to be able to expand up to some % of the available space.
In this particular case, it might be sufficient if I could use the CSS "vw" (viewport width) expression, but that doesn't work either.
What I want is for this to work with a % or better, a means of saying "clamp the image width to a max of the space left after the TOC":
If I use "600px" in place of "60%", it works, downsizing the image to fit in the available space. But as explained in the original post, that clamps the image size on larger screens.