Your image size does not match the canvas size
Every image you upload must have the same size as your canvas size. An image that does not match this size (excluding thumbnails) can cause the image to not display when it is used.
Your image is in the wrong view
You might have uploaded your image in the wrong view. Make sure to upload you image in the correct view.
Your image is hidden under another image
Your image might not appear if it is located in a layer that is positioned under another layer. Make sure that your layer is properly positioned by reordering your layers.