When an Internet-enabled disk image is downloaded by Safari (and Safari’s Open “safe” files after downloading preference is checked), it is automatically expanded into a folder and the image file is moved to the trash.
On Mac OS X 10.2.2 through 10.2.8, this happens whenever an Internet-enabled disk image is mounted (by any program). On earlier versions of Mac OS X, Internet-enabled disk images are treated just like other disk images.
If the disk image has a background picture, it is not transfered to the expanded folder.