The easiest way is to duplicate your image layer and work on this layer. Then, you can adjust opacity and use layer masks.
- To duplicate the image layer, select your layer in the layer palette (if you open your image, it's already selected), then in the menus Layer>Duplicate Layer...
- You can also use the menu Layer>New>Layer from background..., in your case the result is equivalent.
- Your new layer will be selected and now you can use your filters!