If I have an expandable node that has children then the tree uses an image tag with just a style property and no src to create a space between the expand/colapse image and the node text/icon. In IE7 this isn't visible but in IE8 you can see a sunken border around it. This is evident in the Samples Browser on the IG website.
I've worked around this by using some javascript to set all img tags with no src attribute to point to clearpixel.gif but I'm wondering if this is any issue IG are aware of and if it's been fixed in a more recent version (I'm using 2008v1)
I've looked at the samples for our recently released 9.1 and didn't see what you describe. Could you perhaps attach a screenshot. An option is to upgrade to 9.1 and use the new AJAX WebDataTree. This does not use images to create space, so you would not have this problem.
This problem definately occurs in IE8, I'm looking into it now. Those empty images are just spacer images that should not be displayed.