How to detect subjective image quality

Question

For an image-upload tool I want to detect the (subjective) quality of an image automatically, resulting in a rating of the quality.

I have the following idea to real

Answer 1

I'm also really interested working out how blurry a photograph is.

What about this:

1. measure the byte size of the image when compressed as JPEG
2. downscale the image to 1/4th
3. upscale it 4x, using some kind of basic interpolation
4. compress that version using JPEG
5. compare the sizes of the two compressed images.

If the size did not go down a lot (past some percentage threshold), then downscaling and upscaling did not lose much information, therefore the original image is the same as something that has been zoomed.