Oh my, that doesn't look good at all! Does it have raised or rough sectoins? Did your Dr. give you any kind of medicine to put on it? My son got a mysterious rash recently, and he was given 2 types of cream, one was an anti-bacterial and the other was an anti-itch, Mupirocin Ointment and Desonide Cream. Both helped clear the rash up. My pedi also told me to put Lotrimine on it to help calm the skin. A lot of heat rashes looks like that, so can hives, especially if your child has sensitive skin or allergies, which is what my son has.
My mom saw the pic and she says it's hives. She worked in a health clinic for many years and my dad got hives a lot, so she saw hives all the time. In that case, have you tried giving your child Benedryl?
As for the ringworm comment, my kids had ringworm really bad last year they got from a new kitten! If it is ringworm, than the centers of those rashes will go back to skin color and leave a slightly red, raised ring. Two of the rash marks look kind of circular, so watch those, but otherwise, I wouldn't say it's ringworm since ringworm is more in round patches as opposed to a spread like that. If it is ringworm, (which is HIGHLY contagious so everything in your home would need to be washed and sanitized), you can treat it with anti-fungal meds used for athelete's foot. IF the ringworm gets in the hairline thoguh, you NEED to get an oral prescription, b/c creams don't work on hair ringworm and it can cause permanent baldness.
(I looked through your album, what a gorgeous wedding photo, You should be a model!)