The first image that is loaded or created in Krita is automatically assigned as the Stamp in the Brush Tip options. There is currently no way to replace or edit the stamp image without having to restart Krita.
Steps to Reproduce:
1. Start Krita and load or create an image
2. Go to Edit Brush Settings > Pixel Brush engine > Brush Tip > Stamp, and note the image that is being used
3. Load a different image or edit the current one
4. See that there is no way to change the stamp unless you restart Krita
Stamp uses the same image until Krita is restarted.
It should be possible to edit or replace the stamp without having to restart Krita.
I've found that the stamp can be updated by toggling the Brush Style setting from Animated to Regular, or vice versa.
*** This bug has been marked as a duplicate of bug 389432 ***