Images Wrong Size After Theme Move

I have changed the theme on my blog to something new. The trouble is that the new theme I wanted the main content to be 500px wide and then I could use the medium images from Flickr (I am hosting all images of flickr now) to fill its width.

My old theme, that I want to import the posts, comments etc from has a 520px wide content column. Therefore if I import these posts all the images that are the full width on the old theme will mess up the structure on the new theme as they are 20px wider/

Anyone know a way of getting round this issue as I really want all my posts in the same place. Thanks.