Should an error occur while uploading images, the most probable trouble-maker is the metadata section. Open the photo in Photoshop and then review the metadata section. If you locate the problem (metadata content too long in a field? Invalid characters in metadata? etc.), fix the metadata manually and then save the picture in the original format. Retry to upload it again.

If it doesn't help, try to convert the image in the PNG format. Retry to upload it again.