Ed, I dont know if this will help or not but it works for me. You cant have a picture over 800x600 pixels. Most of my pictures are over that so I have to resize it. When I resize it, I dont save the change until I find out how much I have to resize. If you resize in increments, then your final picture is going to be what you resized it to and not the original. To find out if its resized correctly you have to right click on the picture and go to properties. There you will find if it meets the 800x600 pixels.
In other words, when you have your original picture, resize it to where the pixels are good and only save the change then. When you post, it will be small but when you click the thumbnail it will be big. I am certainly not a computer guru as you know but it works for me.
Hope this helps.
Oh, if you want to keep the original as is, then you need to make a copy and work from the copy. If you resize an original and save it to the resize then its like that for good unless you make it bigger again. Hope thats not too confusing.