Dear Leslye,
I’ve been able to reproduce this bug both on your website and on the theme demo by using an IE9 emulator, but after numerous tests the only potential “suspect” that I have is the cache size of the IE9 (there are reports on a very similar issue e.g. here and here). Another fact that supports this conclusion is that the images load well once you are in a portfolio tag archive (by clicking on an icon menu item when outside of the portfolio page), despite the fact that they are built exactly like the portfolio page – see e.g. this page on your website.
One way out of this might be reducing the size and DPI (Dots per Inch, i.e. density) of the images from portfolio, paying special attention to the largest ones. Another (or complementary) solution would be to introduce paging (the “Portfolio items per page” option on the “Portfolio Page” tab of the Theme Options panel) to reduce the number of images displayed on a single page. Please let me know if it helps!
Sincerely,
Andrew / Satori Studio