Hi,
I'm using FastStone Image Viewer for my Screensaver conversions. Its a free program, and you can download it here:
http://www.download.com/FastStone-Image-Viewer/3000-2192_4-10324485.html?tag=mncol
This is how I do it. First go to the folder with the image you want to resize from within FastStone. Then just select an image so that it's highlighted.
Now for the bad news. If the image isn't already close to 600x800 proportinally, (For example it's landscaped instead of portait) you're going to have to crop it so that it's close, or it will get distorted.
So type the "x" key with your image highlighted. Now take your mouse and drag and drop the crop box so that it is close to the shape of the kindle and crop it by letting go of the mouse and hitting ENTER. When you crop it you'll be back to the folder view. The same image should still be highlighted, so hold down CTRL and type "r" This will bring up the resize screen.
On the bottom left of this dialog box, uncheck "Preserve aspect ratio" Then, near the top, adjust the pixel size to 600 for the width and 800 for the height and hit ENTER.
This will take you back to your folder view and your image will still be highlighted.
Hit ENTER again to bring the image in solo view. Now right click on the image and from the context menu that appears, hold your mouse over the word Edit which should be about a third of the way down that menu. While holding your mouse over the word Edit another menu should appear. About two thirds of the way down that second context menu should be the word Grayscale. Click on that. After the colors are gone, hit ENTER. (I believe you don't have to make the image grayscale for the Kindle, it can have color images but will show it as grayscale anyway, but it should make the image smaller, and more room on the Kindle is always a good thing.)
That should bring you back to the folder view again, now click a different image to unselect the one you've been working on and the program will ask if you want to save your changes. Click yes, and there you go. (if you don't have any other images in that folder you can click into another folder and back again and it'll ask if you want to save changes that way)
Hope this helps, it's what I've been doing.
I've been downloading all the cover images of my books on the Kindle via the purchase page on amazon and changing them this way and using them as my screensavers.