My Pitbull has a bumpy rash on his face/neck. What is it and how to treat?

The first picture looks like a large series of insect bites all in one location. If Jackson has been sticking his head in any weird areas where something like that could happen, that would be a reasonable guess as to the cause of the lesions. Determining with certainty the cause of the rash is not easy without significant diagnostic testing including skin scrapings and cytology at minimum. I think that the swelling and the affected area still look bad enough that Jackson could probably benefit from a systemic antibiotic to address the skin, as well as a topical ointment with both antimicrobial / anti-inflammatory properties to help with the swelling and redness. If the area has been itchy even a little bit, Jackson should be wearing an e-collar to prevent any further damage (and as a result delayed healing) to the area in question. As far as things she can do at home if the vet is not an option, use of betadine and a over the counter steroidal cream would both be beneficial. You can obtain an e-collar through your vet or a pet store, or you can even make a home made one ( there are lots of videos available on YouTube on how best to do this). Hope this helps!