I thought it a fake. Looked at the original and went load of bollocks.
A couple of hours later DH came running upstairs with the original, downloaded it, opened it in a photo editing package and selected the background in the highlight colour tool.
You could then see the outline of text that wasnt visible on my screen.
I went through the process of going 'fucking fake news' but DH did prove otherwise to me and im not sure id have believed the story after checking myself otherwise.
So do go to the original tweet and try it for yourself before dismissing it as a hoax. Its not.
Glad it wasn't just me who had this initial reaction.
I'm currently trying to do what you describe, but failing due to (a) inadquate software and (b) inadequate skill on my part.
As I say, I think Boris is a pathological liar and racist pos. I just happen to think that this doesn't mean that everything that gets said about him is true, or that no-one would have a vested interest in faking something like this.
I am pretty surprised his office would do something that monumentally stupid. I suppose it's the photoshop equivalent of screwing up your global search-and-replace in a document.