CR does support PNG (along with BMP, TIFF, JPEG, and WMF), but does not officially support transparency. That's not to say there aren't workarounds, though.
One option is to convert the overlay image to WMF, insert it into the report, and then position the image over top of whatever other image you may have (via the "Move to front" option on the right-click menu). Details are here: Add an image with transparency to Crystal Reports