Even photos holding up signs can be faked, although it's not likely someone would go through that much trouble, but ...
There is a whole industry and infrastructure around exactly providing pictures holding a sign saying whatever you want. Pretty much every time you see a picture of someone holding a sign saying something like that, they were purchased. That's why several years ago people jokingly moved to shoe-on-head pictures, which of course have the same issues, but are funny.
One user recently had a good idea to point to a calendar showing the date while holding the Steemit sign with the date. Another user suggested using videos and I agree that you can't go wrong by doing that and mentioning Steemit.
Its exactly the same issue, you just pay someone to do it. Now you have a dozen videos and pictures with dates of people talking about steemit for a few dollars to put with your posts.
I agree the best method would be a picture with a steemit logo, date and a short 15-20 second video introducing themselves on the post. This will clearly be a great way to verify any user account.