Ok, go to Insert > Link (you can also use the shortcut Ctrl + K) then enter the details in the box. On the left-hand side you can choose what you want to link to (use Existing File or Webpage if your image is either saved on your PC or online). If your image is online, copy and paste the web address into the text bar at the bottom where it says 'Address'. You can also choose how the link will appear in the cell (see the text box at the top), eg 'Click here to view image'.
Hope this helps, but let me know if you have any queries
That was just my example of the text you could enter - you can choose whatever you like. Just enter it where it says 'Text to display' (see the red box below)