Short of (downloading/)uploading as an attachment and inline linking as @BR described, there is no way to resize images (especially existing ones) "on the fly." Mobileread's forum software offers no way to resize displayed images--automatically or manually. You physically resize them yourself with image editing software, or you add an inline link to your post attachments that are too large so they get inserted as thumbnails.
I know of no way to thumbnail images that exist in other Mobileread posts without downloading and attaching to your posts (not to say one doesn't exist). You can
link to an attached image from another post with its Mobileread url (using standard img tags), but it's going to be whatever size the original was.
URL for the above image:
https://www.mobileread.com/forums/at...5&d=1512068749