The last version used a javascript to resize all images proportionally in all browsers, but this script caused several problems, so I removed it. The css I added instead is also causing several problems. The resizing css is exactly like in Twenty Ten (the theme that comes with wordpress), but it is causing problems with the external thumb images (f.ex. video thumb images), so I might have to go back to that script.
I have checked 10 very popular themes, and only Twenty Ten resize the images with css like that. The other ones do not resize at all, some set an overflow hidden, so that images do not flow into the sidebar (or outside the container), and some don't address this issue at all. Users are meant to insert the right size images (use one of the sizes offered by wordpress when inserting), and not rely on resizing..and I agree. Don't misunderstand me, I'm not talking about the default post images (the images above the title and the thumb images) because they are resized by timthumb, and that works just fine.
I'm now working on this issue and will probably recode that javascript, but if this keep causing problems, I will remove it, and add an overflow hidden like the other themes.
btw, in version 1.4 I also added an image shortcode that use timthumb for resizing, so setting a width parameter there can be an alternative if the image is too wide.