Images with the wrong URL

Hi,

I’m having a strange issue at work with a wordpress I only partialy created.
So my work needed a wordpress very very fast, directly online, even without any kind of article or page (yeah … I know, this is not how we do websites…)
So I did my best at copying files on FTP, and installed WordPress the esiest way I know.
Once it’s done, I just create my co-worker an account with admin rights.
He, on his side, installed some pluggins and a theme he paid for. “Newsletter”

The problem is :

Sometimes pictures are displayed correctly with the exact good link :

http://puu.sh/eAzOp/f22b0dbfdd.png

Sometimes pictures are linked to a wrong path … :

http://puu.sh/eAzLT/aab5acd92d.png

—–

I can’t find out where it comes from.
Actually we have checked :

– Theme, tried on TwntyFifteen, without results
– Pluggin, full disable, and removing them from FTP
– Reinstalling wordpress
– Calling web service in charge for the domaine
– Change image size limit on wordpress
– Change database
– Different PCs and Mac
– Even different wifi connexion

And … none of these worked !
I don’t know what to do, we NEED this website s quickly as possible, but I’ve got my back against the wall.
I don’t know what to do next.
It seems possible to me that the theme actually ruin everything, because I don’t know and understand how it works (I usually make my own themes), but on a fresh install, on the FTP, without the theme, nothing worked either…so….I don’t know.

Also it appears that images up loaded from wordpress Admin UI have SOMETIMES problems :

http://puu.sh/eAAen/6dc4e35d80.jpg
http://puu.sh/eAAml/7667f8d0c8.png

Heavy pictures, over ~1Mb I’d say, don’t want to upload, returning a classic “There was a error”, without any precision on a file transfer limit. I’m used to that kind of message, but this one as no details, just “An error occured”.

Also, images SEEMS to be completly upload on server, as in “wp-content/uploads/2015/01” in multiple sizes, as usual. Just the link that should refere to these pictures don’t work

Sorry about the bad english.
I hope I’ve been clear enough to make my problem understandable !