The way I do it.
I have a "flickr" account.
Click on the pic (in flickr)
Click on "all sizes".
Click the size you want.
Under the pic of the size you want is the photo's URL.
Copy it.
On this screen (where I am typing now) you can see some "editing options" up above. See the square with the mountain and the sun above it?
Click on it.
Paste the URL into the space (keep the "http://" highlighted, else it will duplicate from your "copy" step).
Done.