So, install one little Java applet called Picasa2Flickr and you're now cooking with gas. Picasa2Flickr installs a button into Picasa, allowing you to upload any selected images to a Flickr account. You can assign tags and privacy and even resize images before uploading and you can add those images to an existing or new set on Flickr. It's a Windows only plug-in at the moment but the author is hoping to get it working under Linux.
Google's Picasa is an excellent, free application for organizing and photo retouching digital images and pretty much a must-have for any PC owning digital photographer that doesn't have the cash to shell out for a commercial solution so this is a really welcome addition to have for it.
I use Apple's iPhoto and upload images to Flickr using an old, free version of FlickrExport, which works very well. FlickrExport is an iPhoto plug-in that is now distributed as shareware for current versions but you can still find the old 1.3.4 version out there if you look hard enough.
Related Posts: Google Picasa Web Albums For Macintosh, Low Cost Windows Image and Photo Editing.