Here are a few items for you to check:
- check if you have all the permissions on media folder that allow you to do all the actions with it
- images size should be less than 2 MB (since as I know Magento does not upload images bigger than 2 MB)
- check if there is enough space at your FTP for images upload
- check if in image names there are no spaces
- check the sort order of images.
- make sure that during import you assign images to media gallery and that they are assigned as small, base and thumbnail.
- check if your image uploaded to correct subfolders at your FTP.
I can offer you the solution to check images by means of their diagnostics. That kind of diagnostics available in the application called Store Manager for Magento.
Also you can use it to re-upload images. There is 2-week free version of the application, you can download it here -
In case you need any assistance on any of the mentioned above points, please, let me know.
Keep me posted if it helps.