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You can do that yourself.

Load the photo onto your iPhone via iPhoto, sync, pull it up on your photo library, select "Assign to contact"
 
I would love to do this too, but I sync all my contacts via Google Contacts. Is there a Cydia tweak that would allow the full screen pics?
 
I would love to do this too, but I sync all my contacts via Google Contacts. Is there a Cydia tweak that would allow the full screen pics?

If you're using CardDAV to sync contacts to your phone, and haven't applied any new pictures since pulling them down from the cloud, then the pictures you have aren't full screen because CardDAV actually downloads contact photos in low res (or so I've noticed, anyways) and the low res ones only show in a little box in the top right during a call.
 
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