feed dialog: picture not showing up

Question

My Facebook feed dialog link correctly renders a dialog page, but the image set with the picture parameter does not show up -- just the gray line that indicates no picture.<

Answer 1

for me: I had lowercase .jpg in the url string, but the actual image file was uppercase `.JPG'

Answer 2

In case anyone else encounters this issue, make note that Facebook requires your image to an have extension (even if you already use the proper headers). This is true even if your image works in the lint tool without an extension like .png or .jpg.