IMac issues with displaying Internet images

Question

Recently my iMac hasn't been displaying some images on the Internet correctly. For example the header of bbc.co.uk/news looks like a TV searching for a scrambled signal, but the rest of the images on the page are fine. There are a few other sites that I'm experiencing the same issue. When I try to save those images, they download with the same scrambled effect. I'm not sure what caused it to do this, but it also started on my Macbook Pro a few days after. It's not the browser because the same thing happens on Chrome, Safari and Firefox. I've tried resetting the cache and PRAM but to no success. Both computers are running OS X 10.9.
I'm not sure how to attach a file on this forum, but here is an imgur link to a screen shot of the problem http://imgur.com/7hGFSqE.
If anyone has seen something like this, please let me know.
Regards,

Answer 1

Please read this whole message before doing anything.
This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.
Step 1
The purpose of this step is to determine whether the problem is localized to your user account.
Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”
While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.
Test while logged in as Guest. Same problem?
After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.
*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.
Step 2
The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login, by a peripheral device, or by corruption of certain system caches.
Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards. Boot in safe mode and log in to the account with the problem. Note: If FileVault is enabled on some models, or if a firmware password is set, or if the boot volume is a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to boot and run than normal, and some things won’t work at all, including sound output and Wi-Fi on certain models.  The next normal boot may also be somewhat slow.
The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. Same problem?
After testing, reboot as usual (i.e., not in safe mode) and verify that you still have the problem. Post the results of steps 1 and 2.