I know a few people have asked this question previously; however, I think something may have changed recently in the Wordpress update.
I'm building my own responsive theme for my portfolio, and for embedded post images I have added a filter to functions.php to remove the height/width attribute that Wordpress adds to the image automatically. I have used this filter before on other sites therefore I know it should work.
However, no matter what I try I can't get Wordpress to embed the image without the height/width attributes, meaning the image doesn't resize on mobiles/tablets.
Demo for the image in question is here:
Any help would be greatly appreciated.