Ahh, editor only recognizes gif, png, and jpg currently. I could add jpeg to that list. probably something good to add to the config file.
Selecting more than one means it will match more potential slaves. If it matches, then it's more likely to be selected. If however the slave has Big Tits, and the file already has Big Tits tag, then adding the Huge Tits tag will have zero effect on how often it'd be chosen in that situation. If you think something is borderline between two sizes, then pick both. It's mostly useful for hair/skin though where some are so similar, like Red vs Auburn.