How to override CSS image floating alignment default?

Newbie question: My CSS template apparently directs images to float to the right, which is fine most of the time. But what if I have a post with several images, and I’d like ONE of them (only) to be left-aligned instead? I have tried to edit the CSS style sheet and then add corresponding image directives (wrong word?) in the post, e.g. “class=”floatleft” etc., using various recommendations I found online, but apparently I still don’t know what I’m doing. Help!