how to insert images into a message, not from a link, but from a computer?

When editing a post, enter the advanced mode; at the top, there's a menu bar. Click the button with the paperclip icon.
A "Attachment Management" menu will open, and there's a section: "Upload a file from your computer."
Keep in mind that there are restrictions on file types and their size (file size and the total size of all uploaded files).
After uploading, a dropdown menu with the uploaded files will become available on the button with the arrow. You can insert files anywhere in the post.