Gonna be honest, never used ffmpeg for images lol. I take images from PDFs that have transparency and they come out of pdfimages with an opaque greyscale alpha mask and an opaque image. I found it easy to apply the mask with imagemagick, though. Ffmpeg can probably do it but just never had a use case. I just use cwebp to convert because that’s my primary use-case: converting pngs to lossy webp files and cwebp is good enough for me for that:)