You could use a page to post pictures:

You can intersperse the pictures with text and control the size of the picture by left clicking on it and then clicking on the pop down edit tab to change controls on the picture.

You can also go to some other group, main Discussion or Blog post to find a picture you want to use, then click on the picture to bring up the URL path to the picture, capture the URL, then you can click on the Image icon above  and paste in the URL with whatever size

and position setting you want so you can use the picture in what you are explaining.

You also can go to other pages that someone has created and put a link to that page in this page.  You have the ability to go back and edit the page as often as you want. 

You have always been able to use the discussion topic part of discussions for this purpose but this does not carry the extra baggage of the Reply function or of being listed in a Forum.