Thanks Maggie and mch for your response. Good news on two fronts: I now understand your "use rich formatting" comments (relating to the post) and more importantly, with your prompting, have resolved my original issue.
I have now managed to access the Image Tools Format tab by highlighting the image with the shift key and cursor movement. This has enabled me to change the size of the images which is fundamental to creating a presentable web page. The error I had been making was trying to select the image by clicking or double clicking the image to no avail (even selecting the image and some text does not result in the appearance of the Format tab).
My query is now answered and I am making good progress in developing my websites further (until I hit the next "obvious-when-you-know-how" issue!). Thanks again for the helpful advice.