The user-avatar image in the board header is displayed with the wrong "width", because the IE8 is not able to keep the aspect ratio correctly. (see attachment) Should be caused by "max-height" in combination with "width: auto".

Adding a new CSS-rule to "tweaks.css" like:

.header-avatar img

Of course, this will strech images to 25px, if the avatar is smaller. But better than now.