I believe someone with the WIKI_DELETE permission will have to delete the unwanted image files.
I think someone said that the Report spam button when viewing the attachment would also work for deletion after a wiki admin looked at it.
@pierzen: If you upload files that you don't want to be part of the downloaded zip file, you need to use (NOZIP) in the description when you upload the attachment (see wiki:Styles#Iconhandling ). You should be able to replace an attachment's description by re-uploading it and selecting the Replace existing attachment of the same name option.