just download astro file manager, then make a new folder called .pics ( the . makes it unfindable by the phone) then only you would know they were there
Create a folder (name it whatever you want that wouldn't draw suspicion) and put a file in that directory named ".nomedia". Use your computer to just rename a text file or something small to .nomedia
Any directory with such a file in it will not be included in the gallery. Therefor no one would know that the folder contained photos unless they manually opened that folder. Unless your mom is quite tech savvy I doubt she'd even know how to browse the files on the phone without the gallery.