You can change an image's opacity either directly or by adjusting the opacity of its entire layer.
Adjusting the Image Directly
- Select the image on your canvas.
- Find the Opacity slider in the center of the Tool Wheel
- Drag the slider to your desired opacity.
- Tap anywhere on the canvas to deselect the image.
Adjusting the Image Layer
- Open the Layers menu.
- Find the layer that contains your image and tap it, and then tap it a second time to open the layer's options popup.
- Drag the opacity slider to adjust the entire layer.