Updates per #139

[msimonborg]

• Change link values for new images to the context of the image, not the source

• An edit to CONTRIBUTING.md to add this requirement for future contributions.

[hugovk]

Thank you!

When adding link: to the script in #128 I noted the following, but it's not much use hidden away in a closed PR :)

When running, remember to check the URL is the one we want. For example, usually we don't want a Wikimedia URL but a link to the office of the senator (see #125 / #127).

Similarly, in hugovk@930017f#diff-220a71791f0b8551fb315470034e2fd7R2 we don't want a GitHub URL like https://cloud.githubusercontent.com/assets/3171982/7505422/46c2791e-f420-11e4-9360-0bb6ff136a04.jpg but instead want "Not available" as it was sent by email from their office.