If you were developing the theme locally before pushing to your server, there could be a chance that something in the DB hasn’t registered properly.
- Are the images included with the theme?
- Are the images included in your Media library?
Knowing the answer to those questions would greatly help in figuring this out. When you try to upload an image, are you getting an error? It could be that the permissions or owning user of the web directories is not correct. If it is true you are receiving an error on image upload, please paste it here.