I have set the flags for my language selectors to use base-encoding, so the source is something like this:

data:image/gif;base64,R0lGODdhEgAMAPEAACFGi////64cKAAAACwAAAAAEgAMAAACF5SPqcvtr4KctNqLs94W+A+G4kiW5ikWADs=

Now I would like to exclude those from the image sitemaps. I tried this by simply taking a part of the string that is identical for both these images ( R0lGOD ) and set that in the exclude filter for the image XML-sitemap, however, it is not being accepted.
Do you know if this is possible?