For all logos and illustrations, vector format is preferred over raster/bitmap. You can find more information about what the difference between the two is in this article.
Background
Minimum 1920 pixels wide and 1080 pixels high (.SVG, .PNG, .JPG).
Guide Pop-up and Checkpoint
Minimum 500 pixels wide and 500 pixels high (.SVG, .PNG).
Slideshow (Horizontal)
Minimum size of 1920 pixels wide and 1080 pixels high (.PNG, .JPG).
Slideshow (Vertical)
Minimum size of 960 pixels wide and 1080 pixels high (.PNG, .JPG).
Multiple Choice Image and This or That Image
Minimum size of 800 pixels wide and 620 pixels high (.SVG, .PNG, .JPG).
Scenario
Background: Minimum size of 1920 pixels wide and 1080 pixels high (.PNG, .JPG).
Icon: Minimum 500 pixels wide and 500 pixels high (.SVG, .PNG, .JPG).
Portal: Course Thumbnail
Minimum size of 200 pixels wide and 200 pixels high (.SVG, .PNG).
@2x is preferred (400 pixels wide and 400 pixels high).
Portal: Custom Icons
Minimum size of 120 pixels wide and 120 pixels high (.SVG, .PNG).
@2x is preferred (240 pixels wide and 240 pixels high).
