Admin Forum:How to customise Upload Image message?

I'm an admin on the Fastmail wiki, which is plagued with the problem of people abusing it as an image repository. Hence I would like to customise the image upload page so that it includes the message "Images must be relevant to Fastmail — any other image uploaded will be deleted without further warning, and may result in the uploader being blocked". (I've deleted two "get-rich-quick" spam images, and an image of a waterfall that was being abused as an excuse to vandalise pages; for all other suspect images, I've given the uploaders 28 days to come up with a reason why their images shouldn't be deleted — assuming they follow their own talk pages, and if they don't it's not our problem.)

However, I've looked on the "Special:AllMessages" page for the page used to display the upload message, and can't find it; it doesn't appear to be any of the pages whose names start with wikiaPhotoGallery- as I would expect. Annoyingly, the search on the AllMessages page only allows search for page-name prefix (and it only finds a string which occurs at the start of the name, not within it), not for contents; so I can't find the page by searching for its text, and it (unsurprisingly) doesn't appear to be reachable from the standard namespace, even by an admin, so using the general-purpose search can't find it either.

Does anyone know what this page is called, so I can create the custom version? — RobertATfm 12:09, May 19, 2012 (UTC)