Pillow rounded corners: multiply the alpha, dont replace it
Adding rounded corners to a PNG using Pillow when the source image already has its own transparency.
The naive approach — img.putalpha(rounded_mask) — REPLACES the alpha channel entirely, so any originally-transparent pixels in the source become opaque (you lose the original cutouts, just gain rounded corners). The correct path: split the image into channels, multiply the existing alpha by the rounded-rect mask via ImageChops.multiply(existing_alpha, mask), then putalpha(combined). This preserves the sources original transparency AND adds the rounded corners. Useful default for the rounded radius itself: iOS app-icons use 22.5% of the side length (e.g. ~460px on a 2048-px icon).
When masking a PNG that already has its own alpha channel, default to ImageChops.multiply(src.split()[3], mask) then putalpha(combined) — calling putalpha(mask) directly silently nukes the existing transparency.