How do you make sure photos and the captions underneath them "line up" properly. Everything looks great on my monitor, my kids monitors, but several friends had problems with their monitors.

Pictures on top of one another, then words just where ever they felt like being placed.

Assuming you mean the page at:
http://www.salivahfalls.org/pictorialhistory.html

this is happening because the photos and images are aligned using spaces. This means that on some computers which render fonts differently, they don't line up. The best method for creating a page like that is to create an HTML table (with no borders) and put each photo and its associated caption inside a cell.

thanks, I'll try that...

If I make a table for more pics than I have. Will the empty area be visible, other than empty space?

Would different browser automatically resize the cells to fit. Say someone has a 12" monitor set at say 400x600 or other small resolution. Would the cells automatically be sized to fit that screen if there were 3 cells across?

The empty area will be visible since it is a table. I'd suggest only adding another row to the table as it is required, that way you don't have a lot of empty space.

If you make the table size in percentage, it should resize. Is there actually someone still using 600x400? I get accused of being archaic using 800x600.

Dave