Sure thing.

JPEG is the file type best used for real-life photographs because of the way it compresses the picture. However, every time you save the photo it will make tiny little mistakes in the photo that are usually invisible to the naked eye (these are sometimes called "artifacts"). These imperfections congregate around sharp changes in color, which are called "edges".

Every time a file is saved in JPEG, more little imperfections are added. That's the background blue/black noise on this picture. So say this picture was downloaded and re-saved 5 times; that means it'll have five "saves" worth of "noise" on it. If another picture is spliced together with that picture (say, for example, we put a Scumbag Steve hat on it that has been saved 7 times) then it will have more imperfections (more "noise") than the surrounding photo. It will not match. Even if we then save that new, edited photo 3 or 4 more times, the Scumbag Steve hat will always have three "saves" worth of extra "noise", making it visibly different compared with the rest of the photo.

In this case, if the picture were real the whole thing would be more-or-less the same shade of blue. There would be obvious edges and clusters of imperfections around areas of high detail (so more imperfections on the "tattoo" section is to be expected). But, the significantly lighter color around the designs of the "tattoo" indicate that either the tattoo was added completely (which is my guess) or it was simply HEAVILY touched up with Adobe's editing tools. Some other areas that you can see have been edited are the white glows on the sheet, the reflection on the leg at the very far Right edge, where she is sitting directly on the sheet, and the bottom edge of the leg on the Right side of the picture. These all have visible evidence of editing.

The tricky thing about using this tool is that there's no guarantee what the "edited bits" will look like-- it changes from picture to picture. In this case, they're glowing white-ish. In other cases the pattern might not match rather than the color. If the whole image looked white and glow-y, then nothing stands out and it's probably genuine. So you can't say "What bits glow white, those are photoshopped", you have to say "Which parts are obviously different from the photo around it" and that's where changes have been made.

tl;dr: The glowing white bits don't match the rest of the photo, so we can tell it's been photoshopped.