Red X Instead of Picture on a PowerPoint Slide
Most of the time, when you insert a picture onto a PowerPoint slide, you have no problems in the future with that presentation forever showing that picture. The reason is that you have embedded the picture into the slide, so it will always be there.
The down side of embedding your pictures is that this may cause your resultant file size to be quite large, if your presentation is "picture heavy". To avoid this larger file size, and still use a high resolution for your pictures, you can link to the picture file instead. However, that method can have has its own unique problem.
Where Did the Picture Go?
Interestingly enough, only you, or someone else who uses your computer, can answer that question. What has happened, is that the picture that had been linked to, has been renamed, moved from its original location or simply deleted from your computer. Therefore, PowerPoint cannot find the picture and instead places either a red X or a picture placeholder (containing a small red X) in its place.