I have been copying/pasting from my Memoir app without issues until I came to an image. I couldn't get the right-click copy/paste to work. So I got the original image, opened it in Preview and tried right-click copy/paste and I get this:
, the preview windows only starts to shine when you start applying formatting and structure to your notes.
If you aren't familiar with Markdown it is worth reading up a bit on it, there are a few rules you need to remember but once you do I find it really easy to use and write notes.
The guide I linked before and the welcome notes you get when you first start Joplin I think are both really useful in explaining what it is and how you can take advantage of it - even if you end up using the other (rich text) editor it is still worth keeping these rules in mind because that is what Joplin to save all your notes.