Relative dimensions
Relative dimensions allow you to specify a width or height value as a percentage of the main image. This is helpful for features like watermarks and borders.
To use a relative dimension, simply provide a percentage as a number (between 0 and 100), followed by a w (width) or h (height). For example, 5w represents 5% of the width of the main image.
<img src="kayaks.jpg?mark=logo.png&markw=5w">