Two honest ways to fit a page into a feed
Feeds want fixed ratios that comic pages never match. Whole page, padded keeps everything visible on a background (newsprint, white, black, or a blurred copy of the page). Fill the frame crops, anchored to the top, centre or bottom of the page so you can keep the panel that matters; the checkbox hints show how much is lost. Instagram's 4:5 portrait frame is its tallest feed post; the 2:3 Tumblr and Mastodon frame is closest to a comic page and crops least. Everything renders in your browser and downloads as a ZIP of JPEGs.