Charldurand,

Open your style.php file. It's in your template themes. To get there just go to your admin panel and click….

appearance > editor > style.php.

Scroll down to look for this comment mark up

/*– Begin feature section styles –*/

Then find the code below. (It's the first entry under the comment mark up above)

#feature {
    background: url(images/feature-bg.png) no-repeat bottom left;  < this is the line in question.
    background-color: #<?php echo $flex_feat_bgcolor; ?>;
    width: <?php echo $flex_blog_width; ?>px;
    clear:both;
    color: #<?php echo $flex_feat_txtcolor; ?>;
    padding-bottom:10px;
    margin:0px;
}

The bolded image name above is the image of the diagnal lines. You can do 1 of 3 things to remove or "hide" the image.

1. Delete this line completely from the code above. background: url(images/feature-bg.png) no-repeat bottom left;   This will remove all code refrencing the background image in the feature section.
   
2. Remove the image from the images folder. I don't recommend this unless you are comfortable with it.
3. Just remove the file extension (.png) from the image name. Recommended. This option will leave everythig in tact and in place incase you want to retore it in the future.
   

Save your changes and you should be cool. Hope this helps.