I never did say a picture of their mining rigs does negate the chances of them scamming. You are absolutely right though, but how does one use the blockchain to prove mining capability?
If you have x hashpower you should be mining y amount of blocks a day/week/whatever and you should be using those coins that you mine to pay out to your customers. When you mine a coin, you prove you own that address, and use those coins to pay your customers, now we know you are actually mining coins.
It doesn't even need to go into that much detail, just prove that you own one addrss that gets freshly mined coins. Isn't the entire point of all this shit that everything is on the blockchain? Why not use it?