From pptfaq:
Possible solutions

The red X is PowerPoint's way of telling you "I can't display this graphic". Unfortunately, it doesn't tell you why. Or at times it gives you misleading information.

You may see the red x when:

It's a linked image and the link is broken (ie, the image isn't where the link says it should be)
The link points to an image on a network drive or the web, which PowerPoint considers "unsafe"
Memory/resource limitations or other problems prevent PowerPoint from displaying the image
Same article has troubleshooting steps. I wouldn't abandon PowerPoint altogether yet.