Did you want to automatically check the boxes for items already in the cart? Is that what you are referring to? If so, when they add to cart again it will increment the quantity unless you use the option in the add to cart server behavior "If item is already in cart: Replace".
To do that you could add code to the checkbox:
<input name="itemchecked" type="checkbox" id="itemchecked" value="<?php echo($row_WADAProd['ProdID']); ?>" <?php echo($eCart1->InCart($row_WADAProd['ProdID'])?"checked":""); ?>>
The error for the image is probably because the image field for that product is blank or the image doesn't exist. You can fix that by wrapping the image in an IF statement like:
<?php if ($row_WADAProd['ProdImage']) { ?>
<img src="images/Products/<?php echo $row_WADAProd['ProdImage']; ?>" width="73" height="65" alt="" >
<?php } ?>