@sebas974: Login to your WordPress admin. Go to Appearance => Customize => Additional css and add the following css.
img[class*="align"],
img[class*="wp-image-"],
#content .gallery .gallery-icon img,
img.wp-post-image,
img.attachment-thumbnail{
border: none;
}
Let me know if this helps resolve your issue.
Regards,
Tikaram