OK... if anybody ever comes across this thread while suffering the same problem, here's the solution:
1. Use width and height settings on the png files on your login page, or:
2. Use the pngfix.js tweak, but also put:
img {behavior: none}
into style tags in the html header of your login page. This then forces your login page to use the pngfix.js tweak for your login page
's but then applies the pngbehavior tweak to your roundcube pngs.
Problem solved!