One can easily paste the URL of a photo at Flickr into the window provided when one clicks the Add Photo icon in Blogger. But is that the best way to add a photo? One should always provide information about the photographer and the Creative Commons license which allows one to use the photo legally. And this simple copy/paste method does not. It is better to use the Embed code provided by Flickr.
At Flickr.com on the main page displaying a photo, one can click the Share button and choose Grab the HTML code. Then back in Blogger, where you are creating your post, switch to the Edit HTML view. Then Paste in the code provided at Flickr. The resulting photo will provide some attribution. One can hover one's mouse over the photo to get basic info. One can also click to go to the original Flickr page to get more information about the exact Creative Commons license. Below is a test with the same flower picture in previous post.
This result appears to be the same as if one had used the Add Photo icon when composing a post in Blogger and then just pasted in the URL of the small size display page (http://www.flickr.com/photos/24567741@N04/4784562251/sizes/s/). But note that using the Share > Grab HTML method provides a link back to Flickr (as Flickr requests that one do) - by clicking on the photo.
If one does use the simple Copy URL and Paste into Add Photo window, one should at least provide some attribution and a link back to Flickr, for example,
Photo courtesy of cronegeek at Flickr
under Creative Commons License 2.0
Note a similar formulation can be used for Photos from other sites.