Have a page's content ready.
Now for part in deciding what images to use.
I really rather not use images, but eye candy gets more attention than straight text.

If I am to use images, is there a way to add a style to them that allows them to resize according to browser resize?

So when a browser is adjusted, the images scale rather than jump around the page.

The columns in my page are sized by percentages rather than fixed sizes.
What's the best way to go about image placement?

Not through CSS purely, no. CSS was never intended to resize images as such, because browsers were never really intended for that purpose, either.

My suggestion to you would be to keep one column elastic and set the other columns to fixed width. This will allow you to put the pics in the fixed-width columns and even the elastic one if you want (as long as you don't have too much on the same horizontal plane.)

having 3 columns, all elastic,
what would be a good width for images?

Is 200px a good width for most images in all 3 columns?