Why can my phone download some photo attachments from emails but not others?

They're all jpegs and i'm using a k800i and also an LG KU990, both with the same results.

Sometimes i get the error message 'File type not supported by phone' even though they're jpegs like others that do download?

I use a K750i and get the same problem, it seems that the software on my pc doesn't always "kick in" to do the download, if I restart the pc, the software loads and it seems to download all the images.