Sometimes this can occur as a result of uploading your images using FTP, most commonly if you are using the Binary setting rather than ASCII in order to do so. Check this setting if it is available in your FTP client, and make sure that you are set to use ASCII uploads.
Alternatively, your images may be getting corrupted during transfer from your computer to your website. This can happen due to issues on the computer, or sometimes in the network anywhere between yourself and our servers. It could be worth trying to use a different location from which to upload your files, and see if this helps at all.
If you are still experiencing the same issues, contact our support team for more help.