[Logic Ali]

Allows for x or the character you used between the numbers: × (Hex FFFD)

var str = "tumblr_mgn6mfe9aD1rbaz5bo1_500.jpg (500x705)",
    dims = str.match( /\((\d+)(?:\uFFFD|x)(\d+)\)/ ),
    width = 0, 
    height = 0;

if( dims )
{
  width = dims[ 1 ];
  height = dims[ 2 ];
  
  alert("Width=" + width + ", height=" + height );
}