I am subject to legal proceedings where fake iMessages (not SMS) are being introduced by the opposing party and proclaiming to have come from me. I read about spoofing SMS messages. Is it possible for iMessages to be spoofed? If not through spoof, what other ways can someone make a screenshot of iMessages with a fake phone number in the "contact" region?
what other ways can someone make a screenshot of iMessages with a fake phone number in the "contact" region?
A screenshot is no proof at all. As opposed to a photo, you can easily produce a pixel-perfect forgery of a screenshot, e.g. by putting two screenshot halves together, one with your phone number and one with the forged messages.
The chat log files on their machine are no proof either. Manipulating them is trivial as well. A quick search turned up that iMessages chat logs are stored in a
chat.dbfile in the library, using the non-proprietary SQLite format. So, a moderately skilled user can just open the database, change some entries (such as messages or timestamps) and load it back into iMessages. There is no mechanism by Apple that prevents tampering with the logs.
One entity that could maybe prove if these messages were sent is Apple.
If they keep independent message logs on their own servers, these could serve as evidence. But it's unclear if such logs exist and if they would be available to you. Apple claims they do end-to-end encryption, so there is no way for them to verify the message contents either way (although they might know if any messages were sent at all through metadata).
Other than blatant photo editing (online tools exist for this!), it's possible to modify the message database to create seemingly native messages. All data is stored in an SQLite database and while the format is pretty complex (so as to support advanced features such as attachments, interactive apps, etc). This would mean that a sufficiently advanced person could modify their local database and insert messages from a person that never happened. They could literally present their phone to the judge and jury and unless an advanced security analyst were brought in no one would be the wiser.
As far as I know there are no known (or publicly known) exploits which allow iMessage to be spoofed before it is received on the end user device. This would be a major, ultra valuable vulnerability.
With the introduction of stickers you can cover text and make it look like someone said something they didn't. Check out this TechCrunch article about a prank app that does just that.