Missing $index in src() & image() functions

Hi Jonathan,

great plugin, thanks a lot. 

I think i found an issue after calling the `get_single()` function, then using `src()` or `image()` function and passing an index. 
Inside these funtions (line 402 & 424) there is a call to `$this->asset($size)` without passing the index. This results in a default picture from wp-includes.