Once upon a time I scoured the web for the highest quality scans I could find of the so-called disappearing commisar, and, while both pictures are edited—the canal was under construction and dry at the time—when you zoom in it's obvious the picture with Yezhov is more edited than the picture without him.
My conspiracy theory is that he was added, not removed.
Now that you mention it, the lighting on Yezhov's right sleeve does look inconsistent with the lighting on Stalin's coat and there's a faint white around Yezhov's left size :thinkin-lenin:
Actually looked at it closer with my gal pal who's an artist, she agrees: the photograph with Yezhov looks like it very easily could've been a double-exposure to add him in. The light haloing around him is indicative of double-exposure, as is the way that the rest of the picture looks a bit blown out and lighter compared to the photo without Yezhov. It would've been a lot easier at the time to add someone into a photograph in this manner than to cut someone out.
The river is hard to go off of in both — it looks hand-drawn, which was pretty common for photographs at the time as well.
What gives it away for me is how you can see the brush strokes squaring off the railing between Yezhov and Stalin.
The river also seems to have been painted by two different artists, one who preferred a broader, flat brush in the picture without Yezhov and one who preferred a thinner, round brush and more contrasty colours in the picture with him. The area around Stalin is the same in both pictures, and IMO in the more muted style of the first painter.