You currently don't have opto isolation.
For that you must power the Arduino from a different supply than the relay board.
And remove the jumper, and connect relay supply to JD-VCC and relay ground, and only connect the Arduino to VCC and relay inputs (no ground connection between relay and Arduino).
But you might not have to go that far with just the motor diodes and 100n cap on the switchpin.
Didn't read through this post, but it has the pictures for motor diodes.
Leo..
Edit:
The 600mA supply is in theory under-rated for an 8-relay board + Arduino,
which can draw 8*80mA + 50mA = 690mA.
But if you don't activate more than 5 or so relays at the same time it should be ok.