You will need to resize the photos before uploading if they’re very large. (2000 px should be small enough). Once the pic is uploaded,  copy the direct link.

After you’ve created a new post, under the “Text” tab you’ll see a button that says “img” – if they click that and paste in the link, the photo will appear. You don’t need to enter a description when the second window pops up, just click OK.

You can adjust the size of the photo like with any other image, by hovering over the photo with their mouse and selecting the little image icon, then picking what percentage size they want on the left side.