Image Optimization for #1887

I used tinypng.com to optimize these images. With the optimized version it saves around 50% of the traffic. The absolute numbers are not soooo very impressive, but he... ;)
Unoptimized image on the landing page: More or less 110kb
Optimized images on my local box: Less the 50kb.

Is related to this issue:
https://github.com/NuGet/NuGetGallery/issues/1887