Find specific image from global image set?
When you call myGlobalSet.myAssetsField, you're creating an ElementCriteriaModel. This is significant, because an ElementCriteriaModel is basically a "search object".

Your design.imgs call produces an ElementCriteriaModel based on craft.assets. It will return only the assets which are related to your global field.

You can manipulate your ElementCriteriaModel in whatever way you want!

{% set myAsset = design.imgs.kind('image').sourceId(1).first %}