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.