One of our iMac's developed exactly the same problem seemingly overnight.
2010, 21.5" iMac
OS X 10.6.
running VM Fusion, windows 7 on 2nd Screen.
Adobe Acrobat Pro 7 on both machines
Images were barely visible and ghost-like in Acrobat Pro 7 on both environments but were fine in Preview and Adobe Reader. Tried everything down to re-installing and came up with nada. After pulling my hair out for over a week i eventually somehow discovered that by toggling the 'overscan' feature on the iMac in System Preferences > Displays solved the problem. Don't ask me how, or why.
Hope this works for others too.