Nick,
Those plugs look pretty good. You may have a gauge problem. Usually, a misfiring plug will be much blacker than any of yours. A too rich mixture will usually leave the insulator a velvety black color and in extreme cases, a shiny, oily black all over. I believe I'd try John's idea and switch the probes and see if the temps follow the probes. You might also run the engine up to cruise rpm and give it an extra shot of gas with the primer or enrichener (choke) circuit. If it begins to bog down immediately, it may be running too rich. If it speeds up, its too lean.
