Widget images not caching

I’m using ZenCache pro on my website Jardin Culturel it works perfectly, so far, the only exception is that 4 specific images including

http://jardinculturel.com/wp-content/uploads/2014/10/particulier.png

Are not being cached, the only images not cached are in the front page widgets of my theme.

I’m sure I’m missing something simple but I can’t for the moment see what’s wrong.

Thanks in advance for any advice.

James

https://wordpress.org/plugins/zencache/