Review design of image sizes in the timeline

As raised in https://github.com/element-hq/element-web/pull/32626

A review of the current design of images int he timeline is probably warranted

- Screen sizes have generally increased, is there a case to have the default image size be larger
- How should images at different aspect ratios be handled. Should we change from the current 1 size for landscape and another for portal approach. An area-based sizing is another option.

This has been discussed with the design team and it requires further research and consideration along other timeline items(url previews etc).

A design review of the timeline is currently ongoing alongside the work on https://github.com/element-hq/element-web/issues/29971. I've created this issue to track the decision on design and any future implementation.

Start by reviewing the discussion in pull request #32626 and the ongoing timeline design work tracked in issue #29971. Compare default image sizing, aspect-ratio handling, and related timeline items such as URL previews with the design team’s current review. Done means a documented design decision that can guide any later implementation.

Written by the indexing model from the issue text.