custom HTML block for a banner.

Hi,

 

I have a HTML block with a banner code which is size 728x90. the image and link is fine on desktop but when i view on mobile the banner breaks out the block. I have been looking for simple html code to make sure it resizes for small screens but cant find anything that works.

 

Anyone have this problem before or know what extra code to add in with the HTML so banner resizes for smaller screens?

 

see example here for top block (Football Index) - http://gersmedia.com/blogs/view/20/ft-rangers-0-0-aberdeen

 

thanks