Make sure you bring up the picture to view in G-Photos from within a shared
folder - right-click then 'save image address' for the link here (i.e. copy then paste into your post here). I had a few teething problems when I started to use G-Photos also, but soon discovered the 'must be from a shared location' trick! Hope this helps.
An easy way to check if it works is to open an 'incognito window' with G-Chrome browser, which presents you to the forum as a 'stranger' (otherwise you'll be able to see the images anyway as you are their owner in G-Photos....). If you can see the images from an incognito window then so can everyone else - a good quick way to check.