on-line photo sites

I've seen people have their pictures saved on the web.  On here people put links to take you to sites where they have  pictures.  I know nothing about these sites (including what to call them to do a google search).

1.  Do you have to pay?  How much?

2.  How many pictures can you save?

3.  How hard is it to get pictures onto site?

4.  Is there a site you would recommend?

TIA for any help