The best places to look are where you are likely to see the dogs stacked, i.e. set up for the judge's evaluation. The All Breed publications such as Dog News, Canine Chronicle and Showsight are good ones to start with.
For specific insights into what breeders look for, you can look at Breed publications like Dobe Digest, Top Notch Toys, Working Dog Digest and, of course, The Rottweiler Magazine. There are pretty much breed publications for every single AKC registered breed. There are also parallel sites to these paper publications on the web so just type in those names and you'll find a site with some information.
What you're looking for is how the dogs are set up and from what angle they are shot. Keep in mind that conventions like this change over time and you will find, for example, that Doberman Pinschers used to be shot 3/4 from the front and most owners/breeders prefer them shot in full profile from just off the shoulder blade (so the front legs look to be as one). The only way you're going to know what's in vogue today is to check out those publications.
If you're not going to be shooting show dogs, maybe this isn't all that important but knowing how dogs are positioned in their advertising is important when doing portraits of particular breeds even if they are pets. For example, Great Danes are a "head" breed and how the head is shown in a portrait is very important. If you check out some of the advertising in Dog News, you may see some head shots that will indicate the proper angle from which to shoot a Dane.
Jim, I'd start with the publications mentioned above just to give you an idea of how people in the "sport" of dogs want to see their dogs positioned. When it comes to portraits, sometimes doing it different from convention can work in your favor. But I like to say you have to know the rules before you decide to break them.
For some of the other breeds, like Goldens, you need to know whether the handler should hold the tail up (yes) and how about with Labrador Retrievers (no).
For Sporting Breeds, you want ears up; for Hounds you don't want them up. You don't want them laid back either. You want them to hang naturally. The exception in Sporting Breeds are Cocker Spaniels which the handler completely hand-stacks and you don't bait at all.
There are hundreds of nuances which are important for show formals but not necessarily for family portraits. If you ever have any specific question, just ask and I'll do my best to give you an answer.
Jim