Query for `<img>` elements only

While this is probably a case of micro-optimisation, [there is a performance gain of some 5%](http://jsperf.com/queryselectorall-with-attr-only-and-with-element-type) when querying for `img[data-echo]` as opposed to `[data-echo]` only.
Merge pull request #42 from vzwick/patch-1

Query for <img> elements only, 5% perf gain