Erase the grey borders around the images

Hi there, In spite of a lot of researches and manipulations (in the css sheet in particular, and in Wordpress in general), I can't manage to get rid of the grey frame that is put by default around the images I upload on my website. Here is the url of my website: https://geocaching-gizdit.fr/ It is based on the theme Catch box pro. I thank you in advance for any help you can give me. Gizdit