While base Markdown requires HTML code to constrain the size of an image, Quarkdown introduces a syntax extension that achieves the same result more elegantly: !(WIDTHxHEIGHT)[ALT](URL).
The following equivalent examples load image.png with the label Alt as CommonMark Markdown does, but also add a size constraint that fits the visible image into a 300px by 100px box.
!(300x100)[Alt](image.png)
<!-- or -->
!(300*100)[Alt](image.png)
<!-- or -->
!(300 100)[Alt](image.png)
In addition to pixels (whose unit can be omitted), any other size unit is supported. When using other units, you must use either * or a space as the delimiter instead of x.
!(5cm*35mm)[Alt](image.png)
<!-- or -->
!(2.5in 1cm)[Alt](image.png)
<!-- but not this -->
!(2.5inx1cm)[Alt](image.png)
<!-- '%' is an exception. This is valid (although not very readable) -->
!(30%x150%)[Alt](image.png)When you set either component (width or height) to _ (underscore), it becomes automatic, meaning that component is calculated to preserve the original aspect ratio.
<!-- 300px width, auto height -->
!(300x_)[Alt](image.png)
<!-- 5cm width, auto height -->
!(5cm*_)[Alt](image.png)
<!-- 2in width, auto height -->
!(2in _)[Alt](image.png)
<!-- 50% width, auto height -->
!(50% _)[Alt](image.png)As a convenient shorthand, you can omit the automatic height entirely:
<!-- 300px width, auto height -->
!(300)[Alt](image.png)
<!-- 5cm width, auto height -->
!(5cm)[Alt](image.png)
<!-- 2in width, auto height -->
!(2in)[Alt](image.png)
<!-- 50% width, auto height -->
!(50%)[Alt](image.png)<!-- auto width, 100px height -->
!(_x100)[Alt](image.png)
<!-- auto width, 15mm height -->
!(_*15mm)[Alt](image.png)
<!-- auto width, 2in height -->
!(_ 2in)[Alt](image.png)