Something like this should work:
result = "tumblr_mgn6mfe9aD1rbaz5bo1_500.jpg (500×705)".match(/.*\((\d*)×(\d*)\)/
if (result != null) {
var picHeight=Number(result[1]);
var picWidth=Number(result[2]); }
You will probably want to refer to a variable or fieldname.value instead of a literal though. If you just want to store or display that without using it in a calculation, you won't need the Number() function. BTW, result is not a reserved word, just the name of an array that will be created if it doesn't exist already.