Two apps I've messed around with are Photo Privacy (free, with high ratings) and My Secret Folder (99 cents, with high ratings).
Both let you set passwords. Photo Privacy lets you store photos, of course. My Secret Folder lets you store photos, notes, Web bookmarks and such. My Secret Folder also offers a setting to take a photo of anyone who tries to access that app without the right passcode. Of course, iPad 1 has no camera, so that feature won't work on that device.
My Secret Folder's icon appears on your iPad as a folder named My Folder.