I, for one, am certainly showing my own cock. But I have seen others on here who claim as their own a dick pic that has been circulating the porn sites for a decade or more. Hey I guess no harm is being done, let them have their little fantasy.
As for photoshopping, I've seen some pretty professional jobs of that on some of the "big dick " porn sites. But you can usually see through the ruse. One of the most common techniques is to copy a section of the guy's dick, mid shaft, and then add it in to make the dick longer. That can often be detected when you see the vein pattern, or some other feature, being repeated. Most of the doctoring of cock pics that I've seen on this site have been pretty amateurish and obvious. Among the most hilarious techniques is simply stretching the image by throwing off the aspect ratio of the original pic to make the cock look longer, as in the two attached pics of my own cock. The pic titled "normal" has not been altered and in the "stretched" pic you can see that my balls are all out of proportion and the glans is misshaped.
The whole size issue is so subjective and blown out of proportion by both men and women that I've taken to telling folks that I have twelve "internet" inches, that's measuring from the asshole which seems to be the internet standard. Or ten "bragging at the bar with a bulge in my pants" inches, which would be the whole genital package laid out as in the 3rd attached photo. And I do exaggerate my own size just slightly. I always tell folks that I'm 8 x 6 because eight inches seems to be something of a standard for large cocks, and it's a nice round number. I'm actually only 7.75 inches in erect length and right at 6.25 inches in girth at the base. I can't tell you how many men and women have told me that it's "gotta be at least ten or twelve inches!" When it comes to cocks I don't think many people really know what an inch is.