I have a collection of JPEG photos, each 500 to 600 pixels on the longest side. How can I detect which ones have been algorithmically enlarged from a substantially smaller photo?
An online marketplace requires each seller to upload photos of products that it sells, and these photos must be at least 500 pixels wide or 500 pixels tall because product photos with little detail cause a poor experience for buyers. I can already tell if a seller is trying to circumvent this requirement by adding a solid-color border, such as extending the standard white background with more white. But lately, sellers have started to circumvent this by upscaling old photos taken before the 500-pixel requirement was published. What is a good way to determine whether photos have been enlarged with nearest-neighbor, bilinear, or bicubic interpolation?