I added a script to resize the images called imagefit, but I guess you have disabled that one since there was some issues with it. In the new version that I'm working on now I have modified this script to work better though. For now you can enable the imagefit script again, but remove this (some lines above):

jQuery("#container").imagefit();

Then only images in the content area will be resized and it should not cause the reported slowdowns in IE.

but..actually you should not depend on any resizing of images inserted into the post though, the right width preset should be chosen when inserting it (wordpress creates many image sizes of every image uploaded based on the media setting sizes). See step 7 here for how to set image sizes (the content width is now displayed in the theme settings).

Wrecking my brain over why my images don't resize right in IE and FF. Chrome works flawlessly. Any thoughts?

Check it out
http://www.getfitwithandrei.com/20-creative-ads-to-inspire-you-to-get-fit/