PROCEDURE
Use a high-quality camera with good resolution. If your phone camera takes poor quality images, use a dedicated digital camera instead.
Move the horse to a setting with good light. Be sure the sun or light source (an open barn door or window) is behind you. If the light is not excellent and directly on the point of interest, then you should use a flash. This is especially true for skin and eye conditions. Keep in mind that sometimes a flash will spook the horse.
For close-ups of skin and eye problems, touch the screen where you want the camera to focus, right before taking the shot. You can also zoom by pinching and expanding your fingers on the screen, to get more detail and a higher-quality image.
It is a good idea to photograph the problem from multiple angles, because it can be difficult to assess the shape and depth of a problem from just one perspective.
For wounds and swellings, it can be helpful to have several photos from different distances. It can be hard to appreciate the precise location and nature of a problem if images are too close. You want to have a recognizable anatomic landmark in the image.
On the other hand, close-up detailed shots can be helpful too. Its best to have both.
TIPS FOR SAFETY AND SUCCESS
Talk to your vet about their preference regarding this form of communication. Some encourage it, others do not. Some charge for a consultation, some do not. Talk to your vet about whether to send the image to their phone or e-mail address, and alert them that the images are coming.
Always include basic information including your name, your horse's name, age, sex and breed, as well as a brief description of the problem.
Photos are especially helpful for wounds, swellings, growths etc... Take and send a video if your horse is exhibiting lameness, behavioral issues or anything else that is better demonstrated through moving pictures.
One of the worst things you can do to your vet is send a very memory intense photo or e-mail that clogs their e-mail. Use a file format that keeps the image sizes down to 1-2 megabytes, preferably less than 500k, while still retaining good resolution. Send the images separately or in small groupings, depending on their size.
Add additional information about the problem in the e-mail or text message, and be sure to follow up with a phone call, to confirm receipt.