Taille maximale de photos importées

This attachment maximum size has to be lower or equal to the maximum file upload alloted by your server.  If you don't know here is how to figure it out: 

1. Log to your prestashop backoffice
2. Click on "Parametres avancés" or "advanced settings"
3. Click on "informations"
4. Find the answer reading  

    Informations sur votre serveur

5. If maximum file upload alloted by server is too small, then go see your system administrator / webmaster and tell him to increase php.ini.upload_max_filesize to match your requirement.  IF maximum file upload alloted by server is enough, then continue reading below
6. Still in Prestashop > Backoffice > Paramètres avancés  Click on "Administration"
7.  Scroll to "Quota de téléchargement" or "upload quota", update value as you need and SAVE