Flickr
As one of the world's largest image resources, Flickr is ever-changing in terms of its online resolution requirements. Currently, you may still upload high-resolution files to this site, but you can elect to set the online display resolution of those files to a maximum of 2,048 x 1,463 pixels or smaller, should you wish. Again, the real danger of uploading full-resolution files while having them in the public domain is that it might attract unsavory characters wanting to download and use your work for free (see the Copyright symbol section in this chapter). Some folk are happy to do this, but if you are hoping to make a commercial business from your photography, only upload low-resolution, copyright-stamped versions, and store the best, high-resolution work on external drives or use a cloud-based service such as Dropbox.
To get your images out of Elements and uploaded to a Flickr account, it might be best to have the image open (and preferably edited) on the desktop...