Choosing a photograph to work from can take some time, especially if you are trying to find "the" photograph to send me. In order to help you along with this process, there are a few main things I want you to keep in mind.
My general piece of advice is this:
The better the photos, the better the portrait.
Dark and low light photos tend to obscure detail and create blur. Keep lighting in mind as you take or determine photos. Natural light is wonderful! See how much more life Ladybug has in her eye and how much more dimension and detail we can see in her fur? I am able to see individual hairs, whiskers, and the lines in her iris in reference photo 2. These are the details that help me to keep from guessing at what I can't see.
Most photos I receive come from a phone. Try not to use zoom features on your phone, and try not to resize your photos you send me. This can create problems with the clarity, and the photos end up blurry.
This photo of our model, Bandit, took a double hit. Not only does this photo show poor lighting, but the zoom feature on my phone made it even more blurred. Also, it is always best to email photos to me as the actual size instead of choosing "small, medium, or large" in your email options.
I run into this a lot with pets because we tend to look down directly at them. Many of them are shorter than us, after all.
It is always better to send photos taken eye level of the subject. In the photos above, Bandit models two different perspectives. In the first photo, we have a much better shot because our boxer pal is looking at us head on. Photo two, however, would create a more awkward finished product.
The only time I would maybe suggest this is if you specifically wanted an angle where someone or something is looking up at you.
Photos where the subject is in the background or is an "afterthought" are difficult to work from. Here, our Bandit suffers from poor placement, bad lighting, and bad perspective. This is obviously not a photo of him--it is a photo of my desk, and he happens to be in it. References where the subject is included, but not the main subject, usually don't have enough detail to work from.
Lastly, find a photo that you feel captures the essence of your pet, loved one, or memory. You want your portrait to be an accurate reflection of who or what you want to immortalize.
This photo of a dear friend's childhood kitty has wonderful lighting and perspective, as well as clarity in features, and it has the making of a fantastic portrait.