1. Create a new sheet - Sheet2
2. Place your photos in Sheet2
3. Right-click on Sheet2 tab and then left-click on 'Hide' in the popup menu and it will be hidden.
4. To see the hidden sheet again right-click on Sheet1 and then left-click on 'Unhide' and select Sheet2 to unhide.
5. You can hide/unhide more than one sheet.

To be more secure you can also hide photos behind photos in Sheet2. To do this: -
6. Insert the photo you want to hide then right-click on it and select 'Send to back'
7. Insert another photo that you don't mind others seeing then right-click on it and select 'Send to front'
8. Make sure that this second photo is larger than the photo to be hidden and drag the photo over the one to be hidden.
9. To see the 'real' photo simply drag the top photo off the photo behind.