To be honest, she looks more pregnant in the earlier picture than she does in the later ones, guessing she had a pretty bad worm load when you got her?
I don't think she looks pregnant in the later pictures, but it depends on where she's at in her pregnancy. How far along would she be? You said your dad got her in april, and you got her from him later. Assuming you didn't breed her yourselves, that would make her at least in her 8th month if not later. Some horses show sooner, some later, but I'd expect her to be showing more than that at this point. I'd still go for the U/S or a second palpitation to confirm, but from the looks of her I wouldn't be inclined to think she is preggo.