We have an image gallery and are stuck in choosing whether it should be vertical scroll like Instagram feed or horizontal scroll like viewing Facebook photos.
What are the pros and cons of both scroll types?
I think you may be looking for answers where you'll only find vague preferences.
Choosing vertical/horizontal scroll could depend on the following:
Facebook's feed has vertical scroll in its "timeline" or "feed", but when you encounter a user that has uploaded multiple images, or logically grouped ads, they can be scrolled through horizontally. In this example, horizontal scroll is used to logically group content.
Left vs. Right
Backward vs. Forward
Not Attractive vs. Attractive (if you prefer Tinder terms)
Imgur photo libraries are scrolled through vertically. This is likely to create consistency with the desktop implementation. Vertical scroll is "easier" to make responsive, provides consistency across all devices (full-screen horizontal scroll on desktop being less common), and feels "agnostic" of making a decision.
Up vs. Down
Old vs. New
Others may feel differently -- hope this helps.