It looks like you are putting the image on the "Parallax Background" image option.
This option is actually for background like the sky, a city in the distance, or forest trees. It shows where there are empty tiles, so if you have a full map, then you won't be able to see it.
If all you need is a static image, then try the "Show Picture" event command on page 2 (see below image) under the category "Picture".
For weather, there is a different built-in option. But it isn't very flexible. (See below)
I suggest using a plugin instead for weather effects.