How can I make photos fit the exact width of my columns?

I want photographs that I upload to be exactly as wide as the column in my blog. Is there a way to simply (and automatically) do that?

Failing that, how can I find out exactly how wide a column is (in pixels) so that I can insert pictures that match it? I’ve played around with choosing 90% and 80% etc and that sort of thing, but it’s just a guessing game, and I don’t know exactly what the software is doing.

Also, when I then change the setting to 60% or something after I’ve already selected 80%, it seems to be setting it to 60% of the 80% size or something like that, not 60% of the original size. In any event, I get all confused, and I’m hoping there is a simple fix – some kind of box I can check that sizes the photo to fit the width of the column.

Thanks!

Doug