We upload tiny thumbnails of the images we detect immediately, so that you get information as soon as possible. These thumbnail images are extremely low resolution so that they can be uploaded fast (and free) even over very poor connections.
The full resolution images will be uploaded as soon as possible (either by 3G/4G or WiFi depending on how you have configured your device). If the Walker is unable to upload the fullsize images immediately, the thumbnails will act as a place holder until they arrive.
See here for more on image upload methods.