I put together the attached block for you in 6 min using an excellent add-on called Designer Content ( http://www.concrete5.org/marketplace/addons/designer-content/... by Jordanlev).

What the attached block does is adding an image to your area from the file manager (as with a standard image block), but it will only show if the page is in Edit mode, which is the only small change I made from the Designer Content output.

You can grab the whole attached zip and extract to your 'root/blocks' folder. Then you need to install it by going to 'yourdomain.com/index.php/dashboard/extend/install/'.

Now the image block I just created will output the full size of the image, but if you try to create this yourself with Designer Content you can set other dimensions as well.