Let’s say there is a page with 100 different user photo’s shown on the page,
that is at least 100 DNS lookups right there, would this be reduced if I were to link using the an IP instead of a domain url?
http://217.345.33.444/images/photo.jpg instead of http://domain.com/images/photo.jpg
DNS lookups are cached by your computer, so there will only be a single lookup per unique domain.
Additionally, most people use their internet provider’s DNS server, and it will typically cache DNS lookups as well, so a lot of the time, the DNS lookup will just be a single network hop away.
You have no way of knowing when the IP address of a domain will change, so I do not recommend this approach.
Is there a reason you don’t store the images on your own domain? If you did that: