HTML img scaling
Only set the width or height, and it will scale the other automatically. And yes you can use a percentage. The first part can be done, but requires JavaScript, so might not work for all users.
image-scaling
How to scale an Image in ImageView to keep the aspect ratio
Yes, by default Android will scale your image down to fit the ImageView, maintaining the aspect ratio. However, make sure you’re setting the image to the ImageView using android:src=”…” rather than android:background=”…”. src= makes it scale the image maintaining aspect ratio, but background= makes it scale and distort the image to make it fit exactly … Read more