IE can be a bit of a challenge due to the way it interprets CSS and displays web pages, and in this case, I think it is because IE insists on putting a border around any linked image, and when you hover your mouse over the image, the border will actually change slightly (typically thicker) which can make images jumpy.
It would be so nice if IE would simply accept the web standards and make their browsers compliant. As a web designer, I can quickly design a site that works flawlessly in all browsers (excluding IE), but then before I can take it live, I have to go back and tweak the CSS and such so that it will also work with IE. IE8 is better than 7 or 6, but it still is far from what it should be. Still, with the way IE is dropping market share, it might be only a couple years before it is completely irrelevant.
Try this: Temporarily edit one of your posts that has a jumpy image and delete the caption, save the changes to the image, and then update the post. View the post and see what happens. If it cures the issue, then staff needs to tweak the CSS for captions for IE. If that doesn't work, go one step further and change the image so that it doesn't link to the larger image and then check again. If that takes care of the issue, then it is a problem with img a:link in the CSS.
After staff gets back, contact them and tell them what is happening and that it is happening in IE8 and see if they will fix it.