The problem is the "advertising" part.
If a woman posts a random picture on Facebook or Instagram, there's always the chance that some guy is gonna fap to it. But if it's just a normal picture, most people are just going to be viewing it normally, and there will be no real expectation of weirdness. You can have a cheeky fap to the toe-cleavage in your Facebook friend's summer album, and they'll be none-the-wiser.
A fetish image, on the other hand, is made explicitly for the sake of sexual gratification. The mere existence of the image advertises that aspect of it. If someone sees a giantess Photoshop of themself (assuming they know about the fetish), they're going to know what's up.
That's the part that makes it weird. If you start commenting about how pretty your friends feet are or start uploading cropped pics of their toes, they're gonna be weirded out, because you've suddenly started advertising the sexual gratification aspect.