so Im trying out the 9-patch images. I started out with a PNG-file where I have a semi-transparent square with rounded corners:
Make it small as possible. 9-patch cant shrink they can stretch. Fill right and bottom border with black color. Edit 9-patch borders with draw9patch tool to prevent errors. Your 9-patch should look similar to this one.
The 1 pixel border must be either completely black (nine patch markers) or completely transparent. Basically, edit the image to cut out anything in the 1 pixel border except for the two black dots.
Also, your image can be much, much smaller in size. The point of a nine patch is that the markers on the top and left sides mark what is to be stretched. Most of that content with straight edges could be removed.
Try to do 9-patch markings from all 4 sides. Also make sure you have .9.png file extension (f.i. your_cool_pic.9.png)
Here is my example