A comprehensive collection of spider bite photographs can be found at BadSpiderBites.com. In addition to a large assortment of bite photographs, this webpage also contains extensive information about identifying spiders and insects commonly known to bite people. Other pages such as WebMD also have photographs of spider bites.
About.com features a gallery with helpful information about measuring spider bites at various stages. The site also includes information about how to measure an area affected by a bite from day to day to determine if the wound is healing or getting worse.
The most common spiders in the United States that are dangerous to humans are the black widow and the brown recluse, according to Everyday Health.