fix: optimize images size (pytorch#603)

The image on the landing page is about 4.4MB which with some slower
internet connections takes too long also keeping this large file in git
is not ideal.

So I just used tinypng.com to do its job and it reduced about 77%
I think that there shall also be argument to always check all files

https://github.com/pytorch/torchtitan/blob/40a10263c5b3468ffa53b3ac98d80c9267d68155/.pre-commit-config.yaml#L15-L16