Raven is right check that thread out but I think the problem lies with the links you have posted, you have made two requests today. (Breaking the rules I must say
) Both your requests contained a link to a single picture rather than a gallery.
If you add a single pic either by hotlinking, using an imagehost or simply using our own server the preview does not recognise the file extension so no preview. However when you post a gallery link rather than a single pic the preview normally uses the first picture from that gallery.
Edit Re: your pm I've corrected my mistake and given you the correct thread 
it was my bad 