There are a few ways to handle this, but first a little caveat. The F8 Lite theme (https://www.ads-software.com/themes/f8-lite) has seemed to be abandoned for development. Hence the “This theme hasn’t been updated in over 2 years.” message at the top of the theme’s WP.org page. This typically, but not always is a good indicator that it probably not a very stable theme.
A perusal of the code shows that the existing iteration of the theme has no proper check to see if there is no header image then don’t display anything, and that is why you’re seeing an HTML <img> element without a src=”” Without a header image, using this theme the code will not validate, but that does not mean your site will not work fine. It will, but just wanted to make you aware.
You can fix your immediate issue by applying img.headerimg {display:none;} in a child theme. Instructions on how to create a child theme are here: https://codex.www.ads-software.com/Child_Themes
Of course, since the f8 Lite theme has not been updated in two years, it implies that it may never be updated. Therefore, you could just customize the theme itself, which I typically wouldn’t encourage you to do, because an update would override any of your changes. If you’re comfortable customizing the original theme then you could just remove the whole header image block and be done with it, which is this part in the header.php file of the theme:
<?php
$slideshow = $options['selectinput'];
// Only show this is the Single Header Image is selected or if no option is set
if ( $slideshow == 1 || !$slideshow ) {
// Check if this is a post or page, if it has a thumbnail, and if it's a big one
if ( is_singular() &&
has_post_thumbnail( $post->ID ) &&
( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), array( '950', '425' ) ) ) &&
$image[1] >= HEADER_IMAGE_WIDTH ) :
// Houston, we have a new header image!
echo get_the_post_thumbnail( $post->ID, '950x425' );
else : ?>
<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="<?php bloginfo('name'); ?>" class="headerimg" />
<?php endif;
} // End only show if Single Header Image is selected
?>
