Thread:Project:Support desk/Gallery tag and not existing images. Perhaps, a custom placeholder?/reply (3)

I don't know an option or something else to do this. But there is a hook, that can handle this: BeforeParserFetchFileAndTitle. It should be easy to check, if the passed Title object exists, and if not, replace it with another one (e.g. a Title object of an "not-found" image). I haven't found (and don't know any) an extension, that handles this already, so maybe you want to create one? :)