Remove gulp-imagemin and commit optimized images

[wpears]

The bulk of images we serve aren't a part of this repo and the ones that are aren't in constant flux (updates happen a few times per year). As such, the gulp-imagemin task isn't really justified and just causes longer builds and occasional install errors.

Tasks:

• Current source images should be replaced with their minified counterparts
• The imagemin dependencies and gulp task should be removed