The system detects you're attempting to color a picture or to scroll it by your first touch. To prevent a picture from moving please firstly tap the uncolored pixels with a number corresponding to the color you have chosen. Then you can color a picture by swiping it without lifting your finger. It will remain stable. If you tap already colored pixels or pixels with wrong colors the system will detect your action as scrolling.
The best way to prevent moving is to zoom the picture you are coloring. You can zoom a picture by tapping the pixels icon at the right bottom corner.