While Qzzr allows you to crop your images right from the editor, you can also upload images using these specifications below to allow them to be perfectly sized.
Background image:
16:9 (ratio)
1600 (pixels wide) x 900 (pixels tall)
Social share image, and Outcome images (with recommended Facebook settings):
41:20
Question description images:
16:9
Quiz answer option images:
1:1
Poll answer option images support:
1:1 (square)
2:1 (landscape)
3:4 (portrait)
List Item 'Hero' Images:
4:3