@hel_Sinki, I am pretty sure your Discourse version doesn’t have this commit included:
DEV: enhance file type support for theme and component screenshots (#… · discourse/discourse@a76a443 · GitHub.
You don’t need to convert to .png, instead make sure your site is running on the latest version of Discourse.
Which exactly docs are you referring to? Those should be updated to read .jpeg, .jpg, .png, or .webp.