Starting today the brands will appear on the horse image, so you might need to make sure that your brands look nice.

You should keep in mind that the best brands are the simple ones, so it is best to avoid strong colors, or complicated images.
Also, the brands accept transparent images, so you might want to take advantage of that.
Below there are 2 examples of the same brand, one with transparent background, one with a white background:

White background:
http://www.avirtualhorse.com/horse-details/522809/

Transparent background:
http://www.avirtualhorse.com/horse-details/522786/

As you can see the one with the transparent background is blending much better into the image, so maybe you should try that as well.
Also, since the brand image is now part of the horse image, it means that you can alter your brand for new horses, and the other branded horses will stay with the old brands.
This way you can adjust the colors of your brand for each individual horse, in order to match the colors best.

Note: These changes do not apply for horses branded in the past.