I must be missing something, but I can't find where pagination is controlled. I've got one page of over 900 image thumbs and it's all on one page. I see that there is an admin option for images/page but when I set that to a reasonable number, the only page that displays is the first one. I guess it's possible that a previous developer removed the pagination code.

If changing the admin option for images/pages changes the number of items you see, but you are seeing only one page, then the files for your current theme are probably missing calls to the next/back page functions, these draw the relevant links between pages. Are you using a custom or third-party gallery theme?

Try going into admin and selecting one of the default Zenphoto themes, you should then be able to change between pages.

If that fixes things, then you will need to alter the code for your custom theme.

A previous developer had removed the pagination code. Once I discovered that I copied the code from an unused-unmodified theme and it worked great.