Exactly. The only reliable indicator that you see more than the picture really contains or that you get tricked by an exaggerated border around it would be a look at the dimension right next to the display area.
And if something should be changed about the display function, then I would favour a solution that adapts the display object size to the dimensions of the picture so that it never shows any bars around the picture.
E.g. like this for rectangular pictures:
(Pictures would be aligned to start at the top left, not centered)
The reserved area for the picture display would stay the same but the object size would vary.
Or instead of this
like this:
Actually, the longer I thing about it - it may be easiest to start every picture in the top left corner - no resizing of the display area required.
And as soon as the colourful part of a picture does not start top left but somewhere in the middle it has decorative elements around it.
But even this suggestion would not avoid a case like the one in the linked thread:
That could only be detected with a look a the dimensions - which brings us more or less back to square one.