What you'll need :
A Paint Program, or any photo editing program which has an option to "draw" on photos. I'm using a freeware program called Paint.NET
A good conformation shot - for now lets use a side shot, we'll cover front and backs later. (I think there's a sticky on here somewhere about what makes a good conformation shot)
A Ruler and Protractor. I'm using two computer programs for this, but "the real thing" will work too. It doesn't really matter what measurements you get - it's their relationship to each other that really matter. The ones I'm using are BitRuler (Freeware) and Screen Protractor 4.0 (it's a bought program... I think there are freeware ones though!)
Open your photo in the paint program, and draw a box around the horse.
Your box should touch the top of wither, the point of buttock, the bottom of the hooves and the point of shoulder. Like this : (I've tinted the inside of the box... just for creative kicks)
What we want to see is the general shape of the horse. The "ideal" riding horse is a square.
Tell me what you see about the shape of your horse.