Auto Resize images that are too big in posts already made?

Hey all,
I noticed that since changing over to a new theme in my blog, some posts have images that cross over the right sidebar on my blog. There may be dozens of posts like this, and would take hours to go through all of them to resize the posts to make them look good.

Is there a plugin that can resize them in bulk, without having to do all that work?. Or would that be too risky as it wouldn’t be the same fix as doing it all yourself?.