Things you could do to make any image stay horizontal:



Tutorial 1 (fast):

1.- Paste the map on a LibreOffice/Word document, then you put the image in the correct orientation in the screen(enlarge the zoom as much as possible to avoid loosing too much resolution).

2.winOS.- Open the "Snipping Tool" (Windows OS) and cut the correctly oriented image.
3.winOS.- Save the cut. (WARNING: It will be saved as a ".PNG".)

2.- Make a screenshot.
3.- Edit the screenshot so your map is the only visible thing (cut everything else out in the default image program, or you can use GIMP or Paint too).

4.- Post it on Cartographer's Guild and enjoy your correctly-oriented map.

Additional Notes: This method will decrease the final resolution of your image. It's better if you can avoid it.



Tutorial 2:

1.- Import your image on GIMP
2.- Rotate the image with the tool "Rotation" to the desired orientation. (I recommend to rotate in increments of 90 degrees).
3.- Export the image with the parameters you want. You can even choose the type of file to use.
4.- Post it on Cartographer's Guild and enjoy your correctly-oriented map.

Additional Notes: This method should maintain the same resolution as your original image.