Password protected page and upload images

I have password protected one page on my site and now I wonder for how long the page will be “open”? I mean you write the password and then it’s ok to see the page but if this is public computer or a computer used by several people how do I secure that the page is only seen by the person who has the password, i e how long will it take before one will be asked for the password again?
Is there something else I can do to better secure this page?

I use NextGen gallery to present my photos in galleries and this plug-in is great since I can decide the size of the uploaded photos. So now I wonder if I only want to add one photo to my blog – can I use something similar? I want it to be as easy as possible!
Thank you for your h