Easier way to manage images?

I had a question. I’ve been a wordpress user for maybe a little less than a year, so it is still very new to me. I wanted to ask what is the best way to manage images when going from one theme to another. For example I am using one theme that has 2 columns, one larger column for posts and one side column for widgets, etc. I wanted to go to a new theme that has 3 smaller columns and I run into a problem with posts that have images of videos in them that are larger than the column. Do I have to go in and change the size in each post all the time with this issue? Or is there a plugin or someway to get this to change the images?

Thank you.