Looks like you're not actually adding images? If not, forgive the simple explanation below.
When writing your post there's Upload/Insert between the title at the top of the page and the big square box where you're going to add text.
To the right of Upload/Insert there's a little square icon. Click on that and it'll take you through the process of adding images.
Alternatively it might be that your theme has a custom field which you need to create. Somewhere on index.php there might be something loosely along these lines:
<?php
$blahblah = get_post_meta($post->ID, "thumbnail-image", true);
if ($blahblah !=='')
{echo "$blahblah";}
?>
You need to scroll down to custom fields and where it says Name add 'thumbnail-image' (or whatever) and in the value add picture1.jpg (or whatever)