To make this sort of feature and make it look better too, rather than jerkiness, slow reactions when you scroll the scroll bar you can get perfect fixed position on screen, you should use absolute or fixed positioning with CSS on your image. A good place to start is w3schools' tutorial on CSS and positioning:
W3Schools - Positioning[
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with specific reference to the fixed positioning example that does exactly what you want:
W3Schools TryIt - Fixed Positioning[
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However, bewarey of using fixed or absolute positioning as you can get overlapping of elements (explained on W3School page I linked to) and related torubles with different browser sizes - don't set stuff as fixed sizes and distances - use fluid layouts! :)
Hope this helps,
Ed