The 130x70 size was because the image showed up in cockpit images in the top right (I believe) corner. At some point uploading city images was disabled. Then presumably from what Jevon is saying it was switched to the 100x100 image.
So how do you submit the city image? When trying to do it like the other custom images it request the 130x70 image and if you try and submit a 100x100 it has an error that reads "Entity Type not selected".