How do I measure Proportions?
A trained eye can look at an animal in a photo and spot flaws, and get a feel for proportion.  This is NOT always possible in a photo, but the photo can be used as your first basis on whether or not you want a second look at an animal.  Carl Raswan, who did extensive research on Arabian horses, once said that the longer you looked at a bad animal, the more flaws you would see, while if you looked at a good one, the more positive things you would see. 

If a photo is taken from a bad angle, the animal is not standing square, the lighting is bad, it is not going to be a lot of help.  Photos are not the best way to evaluate conformation.  Any animal can be set up to stand square and level.  All of this will change as soon as the animal moves at a walk or trot. 

It is vital in choosing breeding stock (mature animals) that you not only view the animal from all sides (left, right, front, rear) but that you watch it move at the walk and trot, towards you and away from you.  It should be let to square up (or not) on its own after being trotted, not stacked up by hand or with a lead shank.  This goes for ANY size or type of breeding animal, not just donkeys, horse, dogs, cats, or cattle.  A judge in the show ring has very little time to give a good, sound evaluation to an animal, and ribbons that are awarded on one day might not be pinned on another.  It depends far too much on what the judge can see immediately, and What Else Is In The Ring.   A first-place ribbon is not always a guarantee of a quality animal!   Pedigree, parents, temperament, conformation, all must be used in selecting good breeding stock.  The animals being bred now are the future of generations to come.  If bone is lost, conformational flaws are kept, then in the future those will be the norm.  Once lost, many gene traits are impossible to recover.
If you start out with photos, save it to your computer files, or make photocopies.  You will be marking all over them.  Important lines are the three sections of the animal (front, middle, rear), the topline, plumb lines for front and rear legs, angle of croup.  In Miniatures if you are looking to learn about dwarfism or proportion, the length of the head, length of the elbow to ground, and overall length of the back are important.
This is a fairly typical miniature donkey.  The three parts are more or less equal, although the overall length as compared to height shows the animal is short-letted.  The head can also be measured against the forelegs to see if the proportions fall in the .85 or under range. 
The NDMS is the Breed Society for Miniature Donkeys.  Their standard should be used in determining good breeding stock.  They do not use the measurement or comparison system that ADMS has studied on head/leg length for determination of disproportion.  Any animal from registered parents that the ADMS Registry feels should be evaluated for disproportion or dwarfism is sent to a review committee of NMDA.  As per Registry Rules, any donkey with Registered parents may be registered without inspection.

(The Majority of Horse Registries also will registere foals from Registered Parents.  They do not police conformation.  If an animal had crooked legs, the registry would still accept it into the books).