Remember that you can only copy images that are "freeware" or in the public domain. If you have doubts, email the author for permission.

To copy an image: Right-click the image and choose "Save Image As..." and save the image to your disk and then use ftp (see below) to upload the image to your public_html directory on your mason account.

Some freeware sites:
AndyArt (link back to the site to acknowledge the source)
Leo's Icon Archive

For help with FTP:
FTP help (scroll down)