Problem with photo thumbnail on social media

I have a problem when I shared some content from my website to other social media. For example reddit.com or facebook.com

On reddit.com there are no thumbnails...just empty thumbnail window. Please take a look:

On facebook, thumbnail sometimes occured and sometimes not.

How can I fix this? Please advice

google + and Pinterest worked for me from your site.

 Thank you for checking. For me it also works. But on Facebook (does not work always with more photos) and on other websites reddit.com type can not read the thumbnails. Is there any way to fix this?

photos are cached for period of time, for testing purposes, check link below and search newton's posts about this as he's covered this. 

https://developers.facebook.com/tools/debug/ 

 Thank you but this method does not work for me. I have also addthis ant try share with thumbnail and still nothing...sometimes thumbnail show...sometimes not.

I noticed that when many photographs in the gallery (eg here 51 images: http://www.areavis.com/m/humor/view/Figurki-pop-kultury-w-sytuacjach-życiowych-51-zdjęć ) then thumbnail not showed. But when the gallery is about 10 images, then more often thumbnail appears and the choice I have about 3 random thumbnails.

This is weird ... how I can finally fix it? facebook debugger does not help me. It is not just about facebook. It is also about the other portals, where I would place the content from my website. Please advice.