Help notes will be added as the need becomes clear.
The maximum image size is 1,000 x 1,000 pixels and size less than 500Mb.
Adding the image as an attachment is the first step. Having attached the image you will see the button Insert beneath it.
Navigate in the post to where you wish to insert the image and then click on the Insert button below the image. That will add some text to your post which, once saved, will be replaced by the image.