You can select the size and clarity of the media preview in chat screen. There are four options, normal, small, blurred and very blurred. The first two options are for size and the latter two options are for clarity. When blurred or very blurred option is selected, the size of the media preview will be set to small.

To change the settings for media preview:
Go to Dashboard > enter a specific Persona > Tap the setting icon on the top right corner > Chats > Media Preview > Select the size / image clarity
OR
Go to a specific chat > tap “v” button on top right corner on title bar > Info > Media Preview > Select the size / image clarity

These settings are persona-based. The settings for specific chats will override the settings for the corresponding Personas.