I strongly suggest that you get some native English speakers, say from
the UK, Canada, India or Australia to review the record banner.
"Participate in the world's biggest photo competition and help improve
Wikipedia!"
It just sounds odd to an American ear, something about mixing formal
words (participate, competition) with more informal words (biggest,
photo). I've also never much liked "help improve Wikipedia!" It's
more understandable at first reading, but is pretty vague on what
"Help" means. You've got to be careful in asking for help or it will
sound like begging.
For the record, we'll likely use something like the following for the US banner.
"Join in the world's largest photo contest! Historic sites, photos and
prizes."
Pete
User:Smallbones