You can't really lock single apps. Program like you mentioned to hide images or other files are working in their own sandbox/ filesystem location only. If you want to remove stock apps like safari you can use parental control in system settings. That's what I did for the iPad of my son. No safari, no AppStore or in-app purchasing. But the rest is open.
Other stock apps I "hide" in a folder on a screen far to the right. He's not going there.