Hmm, strange. I see what you mean, the windows version of DW CC does not give the Data Sources button.
In this case, what I would do is add a static image, then switch to source view. select the the image src attribute and delete it so the src is an empty string:
<img src="" />
placed the cursor between the quotes, then open the bindings panel, select the itemThumb recordset column and click the Bind button, the image tag should now look like:
<img src="<?php echo $row_rsItems['ItemThumb']; ?>" />
now edit that to add the path:
<img src="images/thumbs/<?php echo $row_rsItems['ItemThumb']; ?>" />