Maximizing picture width in post

I have started adding pictures to posts. I would like to be able to make them as large as the (padded) width of text. But when I try to make them bigger than about 80% of text width, the captioned frame (which I like) stops growing, and the picture sticks out on one side.

Is there a way to expand the maximum frame size, or is there some other issue? I am using floating width of 99%, with two 220px sidebars.

Thanks very much-

Ctenos