Hi Ben.
I use Picasa2 almost exclusively. It is quick and easy and I am lazy. It also sorts my images in a way that works well for me.
This is how I do it. First I use Picasa2 tools to alter the image, mostly cropping and a pass at sharpening. When I get it the way I want it I choose "Export" (button on lower right). I have already created a in "My Documents" named POTN. All images that I wish to post on Potn are exported to this folder.
When the dialog box pops up browse to the My Documents\POTN folder that you have created. By default it will name the new folder the same as the folder name from where the image is being exported.
Move the slider so that 800 pixels is registered.
Here is where it gets tricky. Quality allowed is such that you do not seek the 100kb limit set by POTN. It depends on the size and complexity of the image. It could be anywhere between 50% and 90%. Pick a setting and hit OK. A box will pop up with the new folder in it. Open the folder and hover over the image thumbnail and it will tell you the size of the image. If it is less than 100KB then you are good to go. If it is too big then delete the image and export it again at a lower IQ%. It is trial and error. The trick is to get as close as possible to 100KB as possible.
Then follow Tommy Jensen's tutorial.
Good luck