You would need them to be in focus with good lighting. close enough shot also for details to be seen. A bad picture would be one that is all in shadow so there is little contrast or highlights to make out the details on the horse.
I can offer some suggestions. I do horse portraits and have to work with all kinds of reference photos. Ideally, I like to take the photos myself but that is not usually possible.Ideally, you want several clear photos from several different angles with good lighting. The more the better! I also like to get photos that may not be the best for a reference photo but that shows "who" the horse is to you. His expression, a certain look in the eye, etc that to you says "THAT's my horse". Does that make sense? Sometimes an artist has to work with not so great ref photos and that is where you have to "fill in the blanks" as it were.. with what you know. The same goes for the distortion made by a camera.. a good artist knows how to correct what the camera can distort by what he/she knows about the subject.
We have oils of the girls' two horses (separately, without riders) from their early years. We commissioned them from photos, but the artist knew the horses and the situation and had seen them go many times. She wasn't a professional, just a talented local young artist, but they're some of the most precious things we own 20 years later.
Please remember these are photos I took of the finished product and put on photobucket. The detail in the first two is just amazing. The third pic she didn't have a lot to go on and the pic was 35yrs old and battered.
I sent several photos to Debbie Goldring and told her - I want the pose from this picture, but she's more the color of THIS picture, and can you add her face markings from THIS picture... It turned out absolutely perfect and everyone who sees it loves it. Unfortunately I don't have any pictures of it on this computer... but I HIGHLY recommend her.
If the person taking the photo knows what they are doing (and uses the right lens), the perspective in a photo can be just fine. And a good artist can fix minor problems.
If I didn't paint from photos, I'd never paint!
We're spending our money on horses and bourbon. The rest we're just wasting. www.dleestudio.com
She had the show photographer take several pictures of him and she went from there. I let here pick the angle, ect.. although she gave me the choice.
She is selling one of her early practice paintings of him online (Which I was totally fine with). This one is only an 8x10, mine was 20x24. I origionaly asked for a gold background, but after this one she suggested a blue sky background which does look much nicer. Mine came beatifuly framed, and she had a engraved plate with his name put on it (At her own expense since I won this painting remember). She really gave 110%. I love my painting, and I have always thought that if my house was on fire and I could only save one thing that would be it. I would love to have another painted by her someday of my other horses.
I second the Debbie Goldring recommendation. I had a beautiful portrait done for a fellow Cother and it was done through photos. She is very talented and has the ability to "see" the personality of the horse.