To delete Auras in your account, simply click on the 'My Auras' tab, click on the Aura you wish to delete, and then click on the 'Delete' option which will appear to the right at the top of the list of Auras.
To delete Trigger Images and Overlays from your account, you will first need to delete or edit any Auras using these assets. If Auras have been built using particular Trigger Images or Overlays, you will not be able to delete these assets without first deleting those Auras or editing them so that they are using different assets. Once the Auras have been appropriately edited or deleted, you can delete assets by selecting the 'Assets' tab in the central dashboard, then the relevant Triggers / Overlays option (top left). Again, you can then highlight the relevant asset you want to delete, and click the 'Delete' option which will appear above the list of assets to the right.