I don't know of any sites on the web that have pictures, but there are probably some out there. I have some of David Barrett's books and they have pictures. You hold the mic as you might expect, generally with the lower two fingers of your left hand. Use your index finger and thumb to hold the harp. Grasp the harp as close to the front of the harp as possible so your lips have room. The right hand forms an airtight cup around the harp and the mic. I place my right thumb over holes 8-10 and cap the right side of the harp with my right hand.
I've found that if you really want that "Chicago sound" you have to get a very airtight seal around the harp, mic, etc. This will overdrive the mic element and give more bass response. Small gaps and airleaks make a difference.
As for feedback, I've found that by re-tubing the pre-amp with an AT or AU tube, thus reducing the gain of the amp, gives more volume and headroom without feedback.
Good luck,
Alan
The good times last forever
The bad times fade away