You can remove any visual in a project if you have the leader role. From the participant role you can only remove visuals you uploaded yourself. Viewers cannot remove visuals.
To remove a visual click ‘edit topic’ from the top right of your topic screen, and click the x marked badge on the top right of the visual. You can also remove visuals by clicking ‘delete visual’ when opening the visual itself.
Note that removed visuals remain available in your trash bin for 30 days.