If you receive and error code of 406 or 403 when using the flash based uploader of your store manager it is most likely caused by a server sided module called "mod_security".
You will need to add this code to your .htaccess file to disable it so the flash uploader has the ability to upload.
If you do not wish to OR your site host doesn't permit you to disable mod_security then you will have to upload photos using FTP or the Java based uploader.
Uploading photos by FTP is still preferred by a lot of store owners cause of the ability to let it go overnight to upload 1000's of photos and having an auto-restart ability in the ftp client in case the connection is lost.
You will need an ftp client to upload media by FTP.
Upload the media to the /assets/incoming directory. You can even upload a directory structure of photos.