Small customisation to Wordpress Site

General information for the website: Photography based blog
Kind of development: Customization of existing website
Num. of web pages/modules: 1
Description of requirements/features: I am developing a photography based travelblog, however would like to use the photos I host on my SmugMug site as my featured images in blog posts.

Currently I can only use the WP Media Library when setting a featured image.

What I would like doing:

1. To use an external image (from my SmugMug account) as the thumbnail on the blog posting. So, to be able to set the source URL for the thumbnail.

2. When the user clicks on the featured image the image pops up in a lightbox (with the quality depending on the user's screen resolution). Again the source of the image would be an external URL (from my SmugMug portfolio).

Note: I can source any size image of any of my photos from my SmugMug site (this can be set by using a specific URL which references the size of the photo required).