Check if image has been destroyed when updating dimensions

When an auto-sized image is destroyed immediately after being created (as is the case for Artist blocks that are used to draw the sample level), the `<img>` load tag callback used to auto-size the `<image>` tag would find a null reference in imageElement_.

This encodes the assumption that a `field_image` with no `imageElement_` has been destroyed in an `isDestroyed()` method.

Also sets the loading image placeholder size to be a bit larger.