Auto-crop images in Excerpts?

I’ve tried extensive searching to see if there has already been a post about this, but I didn’t see one, so I apologize if there is!

On my main page, I have excerpts instead of full posts, and I have a main image for each post. I’m wondering if there is a plug-in that I can use that will automatically crop all of my “main” images in each excerpt to 550 x 250. Then, when you click to read the full post, you will be able to see the full image.

I’ve already set the max width for all of my images within posts to 550px, but I’d like to have them essentially cropped to a smaller height (250 px) on the main page so that everything looks consistent.

I hope that makes sense! Thanks in advance for the help!