When setting your size parameter, you can also set it as an absolute in number of pixels in width you want it to be, rather than a percentage of its original size. Doing pixel sizing gives you better control of the resulting size on screen.
To do that use the same command as before but omit the "%" and the NNNN would be the number of pixels wide you want the image to be on screen.
For example if you want your image to be 640 pixels wide:
<img src="URL" WIDTH="640">
Would give you that result.
I'm going to have to experiment with that; I thought the percentage was percentage of pane width -- not of original image size.