Use media tone for picture type

Make `picture` type a media tone, the results being as follows. On fronts:

Before:
![screen shot 2014-10-20 at 15 26 18](https://cloud.githubusercontent.com/assets/813383/4702975/0b0477c4-5867-11e4-9c30-1e23c2bc6ec3.png)

After:
![screen shot 2014-10-20 at 15 26 01](https://cloud.githubusercontent.com/assets/813383/4702981/12e13496-5867-11e4-9b6b-067dfdf78a31.png)

…and on articles, before:
![screen shot 2014-10-20 at 15 24 20](https://cloud.githubusercontent.com/assets/813383/4702989/1b16b820-5867-11e4-9029-5c4e0fbeac9f.png)

After:
![screen shot 2014-10-20 at 15 24 49](https://cloud.githubusercontent.com/assets/813383/4702991/20be3956-5867-11e4-9f78-11dad8b94690.png)