just use drag and drop. Drop it to your post, wait few seconds, then decide if you wanna thumbnail to be seen or big picture.
On other hand you also have option: little square on the right side of smiley icon/ press on that icon named Image, then add link to the image if there is from other site. You can also upload your picture on other site and use link from that. For example:
https://postimages.org/
edit: drag and drop is when you drag with your left mouse a picture from your computer and you let go left mouse when you are with your cursor on the post you are writing.