Never ever take pictures in the barn or at dark with flash! Best pictures are really with the sun in your back, they make it friendly looking too. You are not selling a show horse, so I would put more effort into NICE pictures than super correctly squared up. Half of your potential buyers don't know too much about correct conformation anyway. The trick is: take as many pictures as you can, I take hundreds and delete 98% of them. It's digital, so who cares? Make sure the legs look straight - you filly looks "over the knee" in the pictures. If she is "over the knee" - try to take a picture where it's not that obvious.
I also do not agree in putting all information available into an ad. Of course breed, age, size - the basics for sure, but leave the potential buyer a reason to contact you and ask questions. That is when you can start to ramble about how cute your filly is!
Most answers I ever got on a horse ad was an ad with a neck/head shot of the horse and no more information than registry and age. I did, of course, send plenty of pictures to the interested parties, and she was sold within one week
Now, this works only if you are able to answer e-mails within half a day or less.
Always have more pictures on hand than you put out in the ad. If people ask you the same questions again that were already answered in the ad - don't mention it, just tell them again how old your filly is.
We have the thread "dumbest horse sale ads" here - really, there should be a thread "dumbest horse sale ad inquiries" here too!