Pictures won't load in Safari, or any other browser

I have an intermittent problem where internet pictures won't load. This seems to happen mostly on social networking or picture intensive sites. I've particularly had trouble with myspace, livejournal, facebook, flickr, okcupid, etc., but it is not limited to those sites. Sometimes pics won't load in Safari but will load in firefox. Sometimes the other way around. Sometimes neither of those browsers will work, and neither will opera, explorer, etc. It doesn't seem to matter whether I'm connected by airport or ethernet. I Know it is not my internet connection because I have a PC on the same network that never has this problem. When I travel with my macbook, the problem comes with me.
I am currently running 10.5.2 on a 2 GHZ macbook. Previously, I had this exact same problem on a 550 MHZ Titanium Powerbook G4 running 10.4. I used to think it was just my old slow computer, but when I upgraded computers I transferred all my files and settings with the migration assistant, and the problem seems to have followed.
Things I have tried:
1. Making a new user account. I added a new account but did not delete my old one or do a clean install. This did not work.
2. Using a new DNS server. No dice.
Any suggestions?

Is the iPhone using your Mac to connect?
See if the Mac has any Proxies set...
10.5/10.6 instructions, System Preferences>choose interface>Advanced>Proxies Tab, make sure none are set, like for  HTTPS.