Just wanted to share that the reason my images were showing up as red-exes was because I had spaces in my file name. So, I took out the spaces in the file name (resaved the image with a name without spaces) and they are fine now. (on a related note, linux also is case sensitive.)
Red-x boxes tell you the path to the image is broken to your file name. And spaces in your file name is something easy to check first before you try any more complicated solutions.