Why does my map look stretched and distorted after I upload it?
Open your map in Photoshop, Paint.NET or another graphics editing program and "pad" the image with blank space on the necessary side(s) to convert it from a rectangle to a square. With this option, you get to retain your map as a single image, but at the cost of having (potentially large) areas of dead space on one or more sides of the image. If you elect to use this option, it is recommended that you also utilize the padded area(s) to add flourishes, a stylized border or another aesthetic.
Use image cropping to "trim" one or more ends (or "edges") of your map until it is square. With this option you again keep a single map image, but at the cost of having to cut into the coverage area of the map. It is advised that you review your map carefully before cropping. You don't want to inadvertently remove an area that you had intended (or might want) to use later.
Take your map and divide it into a number of smaller map images that are square or can be made into squares with minimal padding or trimming. Upload each of these sub-maps as a separate map with an appropriate "region" name. With this option, you're not saddled with having dead space on your map (or at least not as much), but the trade-off is that you are forced to have multiple maps to detail what was originally one area.