What will you do with the picture after it is sent to the Uno ?
Will you need to keep full resolution and colors of the picture ?
The less resolution and colors you need to send, the faster it is to transfer.
To transfer anything securely and controllable you need to choose a protocol. I dont know the capabilities of the Uno but I know a bit about sending data over serialport (UART).
- You could try to use one of the established protocol from back in the BBS days: xmodem, ymodem and zmodem.
Could be very fast to implement if you find existing Arduino code for this: https://github.com/ecm-bitflipper/Arduino_ZModem
- Or you could create something simple from scratch yourself.