I experience this too and it seems to happen when you open the image the first time only. It doesn’t happen a twice for the same picture. It also doesn’t matter how far you scroll really, it happens also when the image is fully on screen.

The best remedies against back pain are:

1. Good posture Keep a straight back as much, 90 degree knees and elbows.
2. Excercise Work on moving your back a couple times per week, focus on building back and core muscles and mobility and flexibility of the back.
3. Changing position When possible try to stand up at your desk from time to time, or go for short walks around the office. I like to drink lots of water so toilet breaks allow me to walk and stretch a little.

However if it continues and you seem to be getting warm sensations in your back I would recommend going to a doctor to get it checked out.