Gonna be honest, never used ffmpeg for images lol. I often take images from PDFs that have transparency (rpg books to import into my vtt) 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:)