You cant do that sorry
To see the category you need to let them see the category meaning they can click and see the category. You can only lock them from viewing the photo.
There is no option to do what you wish. You would have to hack your install in some way to lock it down to guests like maybe this in showgallery.php
add this in bold where noted. in showgallery.php I will not support this as this would be considered code hacking. Your free to discuss stuff like this outside product support in the MOD CORNER forums where users can discuss code hacks with one another.
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