Report the bug to Apple using the insect looking button next to the URL bar.
Probably broken JPGs.
Try CTRL + Click -> Show Image.
Or simply uninstall the Beta using the Uninstall Application and it will automatically reinstall Safari 3.
Safari 4 is a BETA dont expect it to be perfect, it is supposed to be buggy.
Some things only Apple can fix, Its a problem with the coding on the browser itself, not your system