The SIY appears normal (before qualifying as a physio I spent 8 years working in an xray department and have some experience of looking at these images). The overlapping joint lines are probably related to the way the x-ray was taken (the diverging x-rays do not pass through the SIY's in the supine position which the x-ray is taken, but get the opinion of a Radiologist with an interest in MS imaging).

You will alwayds find someone to tell you that your Wikipedia reference-linkSIJ's are 'out' 'subluxed' 'rotated' etc but from my clinical experience most problems resolve if you give people a reasoned, non threatening explanation and clear instruction/treatment to address the movement dyfunction. Get your riding position checked and find a recommended physiotherapist. I would personally avoid someone who wanted to rub your SIJ with ultrasound or manipulate it. Hallamshire Physio