You just enter the full web address to the .jpg or .gif file directly into BlackBerry Browser.
If you do not have the BlackBerry Browser or Internet Browser, then your carrier has this feature disabled. You may be able to email the image to yourself as an attachment and open the attachment. (Make sure you have BlackBerryOS 4.0 from the OS4 FAQ at
http://www.BerryFAQ.com )