generate_wallpaper_sizes.py: Properly compress wallpapers

The current options are quite bad and result in files which are unnessarily
big, even bigger than the original file. The Pillow documentation says:
> Values above 95 should be avoided; 100 disables portions of the JPEG
> compression algorithm, and results in large files with hardly any gain
> in image quality.

"100" is not a valid value for the subsamping parameter either.

This change actually enables proper compression with subsampling and also
enables optimization of internal settings.

Result: From 34MiB down to 12MiB with no noticable difference.