Thank you both for your help. Unfortunately right clicking on an image does not always work because the list of programs offered is often too limited. However, I have managed to change the defaults from the control panel as suggested. Having spent so much time on this, I am very grateful to have been given a solution.
Just FYI, if you follow my method, the initial list of programs is small, but there is a "browse" button on the lower right giving you access to all the programs you might want to use to open the files. Many ways to solve a problem. Glad you solved your issue!