how can I save an Imessage conversation ?

I found the following method:

on my iMac messages

marked the conversation and clicked somewhere in it with the mouse

In the Mac menu "Select All"

In the Mac Menu "copy". This will work using right mouse in the conversation too, except be careful not to mark just one word or message by mistake.

Open Mac Notes, create a new note

Right mouse and "Paste"

This copied a very long (months) conversation which included images and also a Voice memo. Everything readable but no date or time.

I also tried the paste straight into a new Pages document and again this was really good to read and even the voice memo was there. Also the date and time stamps were readable.

Hope this helps

-Paul-