rafaelzgz That line you are talking about is part of Flarum's behaviour, it happens if the image is inside a link, just like it happens with text links.
You could remove it by adding this to your CSS:
.Post-body a {
border-bottom: none!important;
}
The problem if you do this is that the line would also disappear from text links, if you don't mind it's the simplest solution.
