Orientation relates to the way an infographic is displayed on a page. The graphic is either read from top to bottom (portrait) or left to right (landscape).
Portrait will be the best choice 99% of the time.
Portrait works best since online audiences are used to scrolling up and down. It is more natural than panning to the left or right.
Portrait infographics also look better in a post view (before they are expanded on their own). More content fits on the screen without appearing squished.
To ensure a portrait infographic will display well on older/smaller monitors stick to a width around 800 pixels wide.
View Full Infographic: Like Infographics? So Do We.
Use landscape only when you have good reason (which you will rarely, if ever, have). It’s a pain to scroll horizontally, and far worse when you have to scroll horizontally and vertically to view a graphic.
(If your infographic will not be used online and is solely for print, then landscape is a valid option.)