Makes the image stretch horizontally. You will need more maps for this.
Use width:number after the URL to use this.
Makes the image stretch vertically. You will need more maps for this.
Use height:number after the URL to use this.
Sets the background of the map. You can choose one here.
Use background:#hexcode after the URL to use this.
NOTE: This is useless unless your image has a transparent background.
The options above can be combined. Here is an example command using them:
/map create <url> height:2 width:2 background:#0b9a7f