I didnt notice a way with the new Google Photos to share an image on a forum...
you still have to do it manually(i.e. it doesn't creat the forum code for you).... From your google photos, choose the image you want and click it to view it then Right click > Copy Image Location - then paste it between the IMG brackets or click the image button and paste it in the URL form(I usually un-check the retrieve file option as well). Like so: