Square thumbnail
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You can create square thumbnail images with the thumbnail option. Here is a 180px thumbnail image:
https://demo.sirv.com/look.jpg?thumbnail=180
Longest/Shortest dimension
You can also scale images to fit a certain dimension, with the s option. The following will create an image up to 400px width or height (determined by the longer dimension):
https://demo.sirv.com/look.jpg?s=240
You make apply that size to the shortest dimension instead, by adding scale.option:
https://demo.sirv.com/look.jpg?s=240&scale.option=fill
These examples show how landscape and portrait images are resized, according to their shortest/longest dimension:
s=240
s=240&scale.option=fill