Do you know if it takes longer for Google to index images powered by Photon?
Images served by Photon won’t be indexed by Google. Google will only index the canonical image, as Photon lets Google know what the original image is. Here is an example:
curl -I https://i0.wp.com/ostseemag.com/wp-content/uploads/2015/06/OSTSEE_Doneiko-2.jpg
HTTP/1.1 200 OK
Server: nginx
Date: Thu, 02 Jul 2015 16:41:12 GMT
Content-Type: image/jpeg
Content-Length: 484925
Connection: keep-alive
Last-Modified: Thu, 02 Jul 2015 16:41:12 GMT
Expires: Sun, 02 Jul 2017 04:41:12 GMT
Cache-Control: public, max-age=63115200
X-Content-Type-Options: nosniff
Link: <https://ostseemag.com/wp-content/uploads/2015/06/OSTSEE_Doneiko-2.jpg>; rel="canonical"
Vary: Accept
X-Bytes-Saved: 31204
X-nc: MISS dca 58
X-nc: MISS fra 20
As you can see, Photon adds a canonical link to the response you get when looking for the image. Google sees that canonical link and can follow it to index the image.
Unfortunately, that’s about all the control you or we can have over the images indexed by Google.
I assume that your logo will indeed be indexed faster as it can be found on all pages. The images you’ve inserted in your posts, however, can only be found in one post, and in the sitemap. That probably explains the difference.
You may be able to find out more by looking at your site in Google’ Webmaster Tools:
https://www.google.com/webmasters/tools/home?hl=en
I hope this helps.