Hi,
It’s not an image, it’s a font. (google FontAwesome)
The markup for the blog element is in includes/page-builder/composer/lib/shortcodes/blog.php
The type of icon to be used is defined at line 162 in there, which is:
$item_icon = "file-alt";
You can check the FontAwesome cheatsheet and replace with a different one (no need to add the fa- prefix). But note that the theme is not using FontAwesome 4, so not all icons are available. I have no idea where you can find the FontAwesome 3 cheatsheet, been searching for that myself.
Regards,
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Cosmin – Support