GetThumbnail() will replace the content of your container field with a smaller copy of the image file. In essence you wipe out the larger image and store a smaller one instead. It may be worth it to keep a second container field in your database that has an autoenter calc that stores a smaller image. Then only use that smaller container field in your iOS file.
If you want to permanently store the data locally on iOS so that it has the fastest load time, then you need to look into a sync solution like MirrorSync that will just transfer the data itself to the iOS device rather than streaming it over WAN. Otherwise you are limited by the file size of the image, connection speed of the client and the upload speed of the server.