Missing Image

the shortpixel image opt. in AO works on all sites, only requirements are you have enough credits (free or not) and that the images can be reached by shortpixel (so not protected somehow). this is confirmed by the fact that it works on all your images except one, actually ??

now why that one image would not work, is impossible to say without more info really, but while flying blind I can point you to this FAQ page which explains how you can exclude an image from being optimized.

hope this helps,
frank