Blogum Theme image sizing problem

Hi,
I have a problem with the otherwise great Blogum theme, the image re-sizing doesn’t apply (I want to insert small images). How can I change that, or where in the css, other people having the same issue I guess?
One workaround would be to work with thumbnails. Here the question is: how can I make them not to crop the original image and keep the original’s proportions, like working with a let’s say fixed width…
I’d be happy to specify, and to get some feedback on this!
Thanks!
ac