Change behavior of app setting which controls rendering of image thumbnails

### Your use case

#### What would you like to do?
I would like the behavior of the application setting that enables/disables the rendering of image thumbnails so that, if disabled, the images are by default "collapsed" (pending [this issue](https://github.com/vector-im/element-web/issues/22113) is resolved). This is different from current behavior, where image thumbnails are instead shown as a normally sized placeholder.

#### Why would you like to do it?
The current behavior of showing a placeholder that is the same size as the thumbnail would normally be rendered is a waste of space that doesn't add to the user experience.

In addition, once a hidden thumbnail has been shown, there is no way to hide it again without clearing the app cache and refreshing the application, which can be slow and introduce performance issues while data is re-cached. This means that if an inappropriate or distracting image is rendered, it cannot be hidden easily.

#### How would you like to achieve it?
See [this issue](https://github.com/vector-im/element-web/issues/22113) for the topic of collapsing/hiding images, and simply use this functionality to achieve a default behavior of hidden/collapsed thumbnails.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_