An open-access website called analyzes digital images to detect potential alteration. One of the techniques offered at the site is Error Level Analysis.

According to the site’s tutorial on ELA:

Error Level Analysis (ELA)permits identifying areas within an image that are at different compression levels. With JPEG images, the entire picture should be at roughly the same level. If a section of the image is at a significantly different error level, then it likely indicates a digital modification …

ELA highlights differences in the JPEG compression rate. Regions with uniform coloring, like a solid blue sky or a white wall, will likely have a lower ELA result (darker color) than high-contrast edges. The things to look for: