I have found that many of the oneshot cover images are that of the scanlated version, not the original language version that the FAQ requires. I think it is the scanlator that is uploading the images but have no proof.
The FAQ can be found here: http://www.mangaupdates.com/faq.html?cid=7#item48
I'll repeat it here:
Quote
Only original cover scans
Only upload cover images that come from volumes released in the series' original language or from the licensed USA volume releases. This means if the series was originally in Korean, the uploaded cover image needs to be from the Korean volume, not the Japanese one.
Only upload cover images that come from volumes released in the series' original language or from the licensed USA volume releases. This means if the series was originally in Korean, the uploaded cover image needs to be from the Korean volume, not the Japanese one.