As mentioned previously, Gimp is a very good open source program for your image editing needs. What is even better about Gimp is its plug-in structure which allows for a greater usage of the program.
One of these plug-ins, called GimpPublishr, will allow you to upload your images to Flickr or Picasaweb automatically. It is quite simple to use, to install it just drop the extracted files inside your Gimp plug-in folder.
The developers have written a pretty good setup tutorial on the project’s page with lots of helpful screenshots.