You can change an image's opacity either directly or by adjusting the opacity of its entire layer.

Adjusting the Image Directly

1. Select the image on your canvas.
2. The Tool Wheel will show an opacity slider.
3. Drag the slider to your desired opacity.
4. Tap anywhere on the canvas to deselect the image.

Adjusting the Image Layer

1. Open the Layers menu.
2. Find the layer that contains your image and tap it, and then tap it a second time to open the layer's options popup.
3. Drag the opacity slider to adjust the entire layer.