If you suddenly find graphics don’t display transparently within your MOSS sites (in fact, within any Web site), then you’d do well to (1) check the format of the graphic and (2) the actual version of browser you’re using to browse the site. If the graphic happens to be a PNG format, then IE6 will have issues rendering the graphic in transparent mode. For instance, the first figure below shows a transparent image on a blue background. The second figure shows the same image but demonstrates how the image, as PNG format, may render non-transparently in IE6. [Images shown below are in GIF format :-) so should show as intended in both IE6, IE7 and FireFox].
This is a known issue, but I discovered it a couple of weeks back while working on a MOSS implementation. Converting the images to GIF saw transparent state in both IE6 and IE7.
There are some suggested workarounds for managing PNG file formats for cross-browsers (IE6 and IE7), which I haven’t yet tried. This one - http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1546508&SiteID=1 - suggests a CSS workaround…