Add images for 2 new blog categories, SixSq/tasklist#26

1. Tech Corner
2. See Through Cloud
Add full images of 2 new blog categories, SixSq/tasklist#26

Big versions (for reference) of the two category images committed before:

1. Tech Corner
2. See Through Cloud

SVG files are also available, but not (yet?) committed.
Merge branch 'master' into lou-experiment
Add support for graphical title
Change category to seethroughcloud
Change to graphical title