Thank you Uma, but that button doesn't show on my reply. In any case, you can reproduce what I did:
1. Create a new project with a button
2. Copy a 16x16 and a 32x32 pixel png files in the project images folder.
3. Add them to the collection of Images in the Solution Explorer panel.
4. Replace the file name in the button smallImage and largeImage attributes in the Config.daml file referring to the new png files.
5. Run the project.
Maybe there is something else that I need to do.
Yes there is a step missing:
after your step 3. you have to change the "Build Action" (under all image properties) to "AddInContent".
Thank you Wolfgang. Now with the steps clearly stated it works perfectly.