Checking the quality

As mentioned below https://whatthemovie.com/shot/440178, what OS and browser are you using?
Because I've just tested it twice and it seems to be working just fine.

Copying what I shouted on your profile a few minutes ago:

Same as ever, nothing changed since my last uploads: ubuntu 16.04 lts; Firefox Quantum 59.0.1 (64 bit) - I haven't had that problem since a few days ago.
I try to crop my shots properly (as you can see), but - of course - I want to check them again in the final upload-state. And that's a bit difficult at the moment...

If somebody else is experiencing the same issue, please report it here (don't forget to mention your OS and browser).

OK, let me rephrase that - yes, I do get an enlarged picture when clicking on it, but it's on a dark background (as usual) - difficult to see if there are any black bars.
Therefore, I used to drag that enlarged picture to a lighter background - when dragged, that picture used to be the enlarged size, now it's as small as the non-enlarged picture.

Sorry for not having explained my problem clearly before.
And many thanks for all your efforts to solve it.

I just tested that using one of my Awesome Uploads (just to that final state). Same problem.
Maybe it's a glitch in Ubuntu or Firefox as you suspected - I didn't change anything.

Was it always like this, or is this a new situation?
I just tested, and it works for me like it always does: first you see the picture in the small size surrounded by black, and when you click it it gets filled up to WTM-size in red (not black, so black bars should be visible).

Maybe the dark background greyhound is talking about is what's around the filled-with-red WTM frame:

@greyhound: Have you tried right-clicking on the enlarged picture and then choosing "View Image" or "Properties"?

@MisterZob

Yes; that's what I meant - black lines to the left or right are hardly visible there.
That's why I used to drag the enlarged shot to the white background, but that doesn't work anymore because it switches to small then.

Thank you for the "View Image"-hint - its' fully enlarged there; black lines to the side should be easily visible -
and the background is dark gray there; it should be no problem to see black lines there. Excellent!

Thank you again for your efforts!