Recompressed every single PNG losslessly

I have compressed every single png on the website losslessly. This means that although the image has lost size, the images are **pixel-for-pixel** identical.

Summary of size changes:

assets/imgs/LockOnly.png 10.479 kb > 2.88 kb ~27% of original size
assets/imgs/logo_head.png 2.029 kb >  1.023 kb ~50% of original size
assets/imgs/logo_main.png 8.187 kb > 3.863 kb ~47% of original size
assets/imgs/screens/screen-osx.png 178.73 kb > 67.465 kb ~38% of original size
assets/imgs/screens/screen-win.png 178.731 kb > 68.76 kb ~% of original size
assets/ss.png 151.34 kb > 44.045 kb ~29% of original size

Total: 529.496 > 188.036 ~35% of original size

Reduced the assets folder by 341.46 kb.

This cuts the size of the entire project from 860.3 kb to 518.8 kb which makes the project 60% of it's original size. Merging this pull request, would basically double page load times and cut bandwidth costs in half.

The tools I used were optipng and advpng, both are FOSS.