PROCEDURE
Use a high quality camera with good resolution. If your phone camera takes poor quality images, use a dedicated digital camera instead. Use a flash, but be careful not to spook the horse.
Move your horse to a setting with good light. Photograph the problem from multiple angles, because it is difficult to assess the shape and depth of a problem from just one perspective.
Take several pictures from multiple distances, zooming in and out. It is hard for a vet to understand the precise location of a problem if images are too close or there is no other anatomic landmark in the image. On the other hand, detail shots are helpful too.
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.