Error Level Analysis
Show which parts of a JPEG have been edited, by finding the regions that compress differently from the rest.
You can also drag an image here, or press Ctrl+V to paste a screenshot.
What the bright patches mean
Saving a JPEG throws information away, and saving it again throws away a little more, but only a little, since most of the damage is already done. A region that's been pasted in has usually been saved a different number of times from everything round it, so it still has more to lose. Re-saving the whole picture and subtracting the result makes that region light up.
Read it carefully. Sharp edges and strong texture are naturally brighter than flat areas, so brightness alone means nothing. What matters is a patch that is brighter or duller than similar material elsewhere in the same picture: text that glows against a background of comparable detail, or a rectangle whose interior differs from its surroundings.
This only works on JPEGs. A PNG is lossless, so re-saving costs nothing and the difference is uniformly black; the page will say so rather than pretending otherwise.