It looks to me like she just needs some exercise! I have a thoroughbred and he looks similar to that, he has a hard time keeping a top line. There are weight builders you can use but they don't always work in the places you want them to! Canter work helps them with their topline as well as trot poles. You can also do a lot of hill work. As far as a weight supplement though, I had my horse on something called Fatcat (on smartpak.com) that worked great, he was off of it in a few months though because then he just started getting a big belly, but if you use it with regular exercise then that shouldn't happen. (I was pregnant and wasn't riding!) It's very typical of thoroughbreds to be a little bit ribby - but there is a point at which they are definitely TOO ribby, if that made sense. My horse is a little overweight in the belly due to lack of work, but his ribs will always be a little visible!