I made a small script that scans the iOS contacts list and looks for contacts that have a Twitter handle set in the Social section.
For every Twitter username found, the app will offer you both the current image and the Twitter profile picture. Then, you can select which image you'd like to set to the contact.
You can get the script from GitHub.
Currently, the script will suggest the Twitter picture, even when it is identical with the one from the address book. However, I'll add support for detecting if the images are equal soon.
This is what it looks like:
(I know that the code probably could be written in a better, more elegant way, but it is my first Python script and I think it's definetly good enough to share it. If you have any suggestions how it could be improved, please let me know)