Custom image background for spaces

### Your use case

#### What would you like to do?
Having the ability to add a custom image background to a space, similar to adding a custom image banner in a Discord server

#### Why would you like to do it?
It's nice being able to customize a space more

#### How would you like to achieve it?
Add a custom image background to the 'explore space' screen

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

Start by locating the Element client entry point for the “explore space” screen and reviewing how its current background is rendered. Define the supported custom-image behavior and verify that the screen displays the selected image as the space background.

Written by the indexing model from the issue text.