text wrap around causes image to overlay my post footer

I finally figured out how to get my text to wrap around an image aligned to the left (by pasting some code into my style.css, which is a letterhead template with barely any image coding in it originally). But what happens now is that the last image (or a tall image) will always overlay not only the post footer (with the “leave a comment” etc), but also the previous post below it.

What’s up with that? And how do I get it not to do that? After doing a little research, it seems to have something to do with a div clearer, but I’m not sure how that works, and if including code in the css is enough, or if I need to do something html in my post itself (and where to place it).

Anyone else have (and solve) this problem?