Support different image quality

Downloading the big raw images whenever open a document can waste a lot of bandwidth, as most time people just need to see the image snapshot but not the raw/high-quality version.

Can we have some strategies like:

For big images(e.g. >1MiB), use a smaller version as default (e.g. ~500KiB), and load raw images when users click and view them.

This can save a lot of bandwidth.