By default after being sent to the website, data is saved on the mobile device for 10 days, then automatically purged. This includes media files. You can change the 10 day setting to: Never, Immediately, 10, 20 or 30 days.
To change this setting, open the doForms app on the mobile device and click on "Settings". Alternatively, this setting can be controlled centrally for all the devices in an account from the website by going to Manage > Account > Mobile Device Settings.
The location of this backed up data on the device will depend on the specific device. But it is always a "doForms" directory on the internal flash file storage of the device. doForms does not currently reference external file storage devices such as microSD cards.