If you use the GIMP to retouch and tweak your photos before you upload them to Flickr, then you'll most certainly appreciate the GimpPublishr plugin. It allows you to upload the currently opened photo to your Flickr or Picasaweb account directly from within the GIMP. To install GimpPublishr, download the latest version of the plugin from the project's Web site, unpack it, then move the resulting libpub directory and the publishr.py file into the .gimp-x.x/plug-ins folder in your home directory.

Launch the GIMP, and you should see the Publish on Web command in the File menu. Run the command, and choose the service you want. If you choose Flickr, you'll be prompted to authorize the plugin to access your Flickr account. Once you've done that, give your photo a name, provide a short description, specify tags, choose the desired license, and hit the Upload button. That's all there is to it.