WordPress Importer rejects some jpg’s

I have just discovered conclusively that the WordPress Importer rejects certain jpg’s – arbitrarily, it seems. In other words, out of a list of about 20 jpg’s referenced in an exported xml file – which look perfectly normal when viewed – it accepts all but one. Then If I take one of the ones it accepted and re-name it to the name of the one it rejected (without changing anything else in wordpress or the xml file), the uploader accepts it with no problem. So there is obviously somehthing about the jpg file itself that the wordpress importer doesn’t like. It isn’t the size; the file is actually relatively small compared to many (but not all) of the others.

There must be someone here who knows enough about jpg’s to help me understand what is going on (or who knows something relevant about how Image Uploader handles jpgs).