OK folks... here something new to try.
I'm a programmer. After suffering this on and off for over a year; after trying everything here, and finally, after looking at this and testing ideas for three hours today, I came to the conclusion that it was not some application that was causing the issue (at least for me) but some preference file.
In my current situation, the blurry icons are always on mounted .dmg's, and are always applications. After knuckling down, and finally reaching the "it's a preference file" conclusion, I did this:
FIRST: quit all running apps.
Next, went to my user/library/preferences and dragged
com.apple.frameworks.diskimages.uiagent.plist
com.apple.preference.desktopscreeneffect.plist
com.apple.DirectoryUtility.plist
out to the desktop.
Then, viewing as a list, I sorted the display by size, putting all the Zero Length files at the top of the list. Then >SKIPPING THE FOLDERS< I hightlighted all the size zero files (which were all lockfiles) and deleted them.
Then I immediately rebooted. The fuzzy .dmg icons are gone, replaced by the correct sizes and resolutions.
The lockfiles are just directory entries, not real "files" occupying space on the disk (hence the zero length). I had over 1500 of them. They are used only to indicate that another file is open and in use, and therefore should not be opened by some other program until the first closes and releases it. (That's why I said to quit everything first, and then reboot after deleting them.)
As to the three plist files... hey, I did that at the same time, and things are working for me, so I left it in the explanation.
>> I would -strongly- suggest that, like with any such sweeping change, you HAVE A CURRENT BACKUP FIRST!
My speculation is that some system preference got "locked" (according to the lockfile) and thus could not be updated, resulting in either the finder, or the DiskImageMounter being unable to determine the proper resolution (or, who knows, maybe it was one of the plists...)
As with all the hundreds of other posts here, this may or may not apply to your situation, and if not, you have my sympathy.
But perhaps this will work for someone else besides me.
Good luck!
Tracy