If you are considering commissioning a painting, there are a number of points to consider.
You will need to have good clear photographs with good lighting. Strong sunlight can create too much contrast and obscure information about the horse. Soft sunlight is best if possible. Flashes are not acceptable. Professional photos are excellent providing permission has been obtained from the photographer and no copyright laws have been breached. I am happy to take photos if the subject is within a reasonable distance from my studio, otherwise consideration of time and travel is requested and can be added to the fee.
A good idea of composition, for example, full body, head study, action and background, need to be considered. Also framing expectations and size. Prices are variable due to a number of factors.
A deposit is required prior to commencement, and you, the client, are responsible for the freight costs.
I also like to know about the subject and client, for example, which area of horse activity you are interested in, time frames, your location, what relationship you have to the subject, and why you chose to have the portrait commissioned.