It seems a few people for some reason are getting that, I don't know what the cause is, and I cant replicate it. It doesn't seem to be related to the ammount of downoads, during testing, I've scraped 1000+ movies at a time and not had this issue.
From the next version people will be able to select from 4 sources for movie posters, namely MoviePosterDB, Internet Movie Poster Awards, The Movie DB, and IMDB.
They will be able to select the order that MC tries for posters, if the first choice doesnt have a poster then try the second etc.