This will be something in the Options of Outlook Express.
I don't have it to hand to check exactly where, but go into Prefrences/Options, and check for allowing images to be shown in e-mails. I'm sure they'll be an option.
You see, with slower modems, this option would be okay, as it wouldn't download the picture information to make e-mails upload faster, yet of course some e-mails have picture information because people have sent them purpously, in which case, its best for people to sometimes have them as attachments just in case.
So when its downloading, its ignoring the pictures and removes them from the e-mail, leaving an empty space.
Find that option, switch it off, and on the next set of e-mails, you should have images
I'm afraid the e-mails you've downloaded though probably won't get them, because it wouldn't have downloaded them in the first place