The simplest way to use it is to select Catalogue Search from the left hand list. Then enter the text you want to search on in the top box marked “Any Text”.
For example if you put in the search term Balerno Post Office you get 9 matches (it displays them five per page by default but you can change this). Select the ref no of the one you want and it will give you the details.
You will see in the example that the third one down (CHP/4/1/30) has a thumbnail image which you can click on to get a slightly bigger image.
The high quality images are not online. Others also have thumbnails but not them all.