From the Model Mayhem FAQ:

Help! My images are uploading sideways! What can I do?
Model Mayhem doesn't alter your images when you upload them, except to resize them if they are too large.

However, some users have noticed that images will sometimes appear rotated when they upload the image to Model Mayhem. This is likely a corruption of the image's orientation data in the image source.

You can easily correct this by renaming your images as or after you download them, which should force your image's data to refresh to the correct orientation data. If you do not want to rename your images, you can try rotating your images in an image editing software.

If you are still having issues uploading your images, you will want to investigate your images in an image editor to ensure the orientation is correct.

Here are some resources from the internet that might help explain why your images aren't uploading correctly: