basic bitmap storage
Useful for comparing two JPEG images saved with a different compression ratios.

You can use these pictures for testing (use the full-size version of each):



50% quality JPEG

100% quality JPEG







link to full size 50% image

link to full size 100% image

The expected difference for these two images is 1.62125%

img50 = ImageData@Import[NotebookDirectory[] <> "Lenna50.jpg"];
img100 = ImageData@Import[NotebookDirectory[] <> "Lenna100.jpg"];
diff = img50 - img100;
Print["Total Difference between both Lenas = ", 
 Total@Abs@Flatten@diff/Times @@ Dimensions@img50*100, "%"]