I was thinking of it as a play on the word "free" in french before the word Stock...

    Gratis - Free as in Beer
    Libre - Free as in Speech

Something else with a hilariously goofy name that will limit its potential.

I think they're referring to the fact that you using the word "stock" in your name is a bad thing. I.e. searching for GratisStock just like LibreStock both yield search results that take me to the "big" stock-image sites, with the keyword being search. I.e. Stock images for keyword Libre, and Stock images for keyword Gratis.

Hopefully it changes if your site picks up steam.