Hi Andres
To see where the problem lies, you could try this:
1. In Visual Studio, create a simple add-in with just a button item using the SDK Templates.
2. In your file explorer, access the Images folder of this add-in on disk.
3. You will see the GenericButtonBlue32.png and GenericButtonBlue16.png.
4. Delete them.
5. Copy your custom png to this folder and rename your png to be GenericButtonBlue32.png and GenericButtonBlue16.png
6. In Visual Studio, compile your add-in project and test if the button loads in Pro with the custom png instead of the generic blue image.
If it does, then you could check your add-in where it is not working. If it does not work, it could be a png issue.
Thanks
Uma