Compress images

@thomaspark

I personally would go with the 8-bit images. I see no visual difference apart from htmlandcss.png which now I actually find looks better; the black looks darker now.

If you don't want the 8-bit images, let me know and I'll rebase the branch.

Total size difference with the 8-bit images

```
Before: 946 KB (969.290 bytes)
After:  405 KB (415.638 bytes)
```