Dave, what I do with my images on Flickr is, I click on it until it displays on its own and then right-click on it to bring up the menu with the choice of sizes (Medium 500, Medium 640 and Large on my FF). In that menu, I left-click on the size I want, wait for the photo to display, then right-click to "Copy image location" and paste the url in the Image box in this site's reply.
Trying it now:
Click on photo on Flicker's list of thumbnails
Photo appears larger but with magnifying glass symbol in place of the cursor. Click on photo.
Photo appears again as part of (static) slideshow
Right-click on photo, choose Medium 640 from menu
Right-click on photo that appears, to bring up another menu, choose Copy Image Location
Click on Image icon at top of this reply box
Paste
OK
Result: