Ajibel:
Thanks, but I do this already. Actually I copied bootstrap's code for my own solution which goes this way:
img { max-width:100%; height:auto; vertical-align:middle; }
*cant remember the rest now*
But then this would still load the exact same image for any screen res. Imagine your desktop header image is 40kb, same img would be loaded for mobile users which would eat up data and slow down the time taken for the page to be rendered on a small device.
You are not yet on "beginner level".. Pls train harder for your sake.
He used width not max-width so try using width or set a min-width and max-width...
Or use media queries..