Open the Find a Grave app and Sign In. Navigate to your Profile (tap photo in the upper right) and tap Edit. Check the box next to 'Upload on WiFi only.' Tap Save. The photos will now save to the upload queue and only upload over WiFi.
To find and upload the headstone photos once you are on WiFi tap Contribute from the menu and then Uploads Queue. If you added the photo from Add Headstone Photos on a cemetery page the photo will be under Transcribe, in the same menu at the bottom of the app.