My dog's kidneys are small on ultrasound. Is it kidney failure?

I wouldn't say that Angel's kidneys are negligible. They are still, though small than "average." This could be her normal (not all dogs will have organs of the average size, since a 10 pound dog will have smaller kidneys than a 100 pound dog). Other causes for small kidneys could be a congenital disorder, fibrosing or scarring of the kidneys, significant dehydration, among a few other things. Ultrasound still images can be very hard to interpret since you need to scan through things to be able to get the whole picture. From the still images, either they do not capture the actual kidney, or there is so much damage that they are not a normal appearance anymore. Even though this link ( http://www.ultrasoundpaedia.com/normal-kidney/ ) is for humans, dog kidney images will look very similar. You can see a few different shapes, and the odd shapes seen in the pictures in the link are not seen on your images. Causes for loss of architecture (kidneys that are more or less mushed all together) can include infection, scarring, cancer, etc. Even though she is young, it is possible for Angel to be in kidney failure. Bloodwork and urinalysis would be the tests to show this. If she is in failure, then she would need dialysis to continue with life.