Check this topic which asked the same question -
https://forum.avast.com/index.php?topic=324965.0A google search for your topic title returns many hits -
https://www.google.com/search?q=Difference+between+Avast+One+and+Avast+Antivirus+Free- Attaching Images to your post - When you Click the Reply button it opens a text window for you to post your comment (reply or post).
Click the Preview button, that shows what you have input and expands it to include 'Attachments and other options'. Click that it further expands, here you can attach images, etc. at the bottom of your post.
See my attached image, click to expand.
I don't use Avast One so can't provide one. Again a google search for Avast One FAQ will surely give that information.