Hi albopictus,
it is quite easy to change the cities:
Just go to the plugin directory of ankiwarrior and open the aw_cities.py in a text editor. There you will one line for each city, like this:
1 : (u"Kōnan", u"江南市", 100200, "http://ja.wikipedia.org/wiki/%E6%B1%9F%E5%8D%97%E5%B8%82"),
where
1 is the level
Kōnan is the city name
江南市 is the city name in Japanese
100200 is the population of the city
and the last one is any link, in this case to the wikipedia site of the city.
Just replace this information with the Spanish cities. you can remove all entries from 51 to 267, if you need only for example only 50 cities. Just make sure you'll have a continuous number range starting with 0.
For the images, just replace the files in the ankiwarrior/img folder with whatever images you want. They can be gif, jpg, svg or png. Just make sure the name of the image equals the rank of the city. The image file for the above example (Konan) should there be something like 1.jpg or 1.png.
To create new google maps images is unfortunately not that easy. I had a list of coordinates of all the Japanese cities and wrote a program to automatically generate the google maps images with it. I don't know if I still have that program somewhere, if it is still working and if I still know how to use it. I would therefore suggest you try to find some nice images :-)
Let me know if you have any further questions or any problems with editing the above mentioned file.
Proxx