If you have a windows based pc, then you have a program called "Paint" in the accessories section of your programs. Run Paint, open the picture you want to upload, select resize and then either indicate you want to use 640 by 480 pixels, or just adjust the size until it is what you want. Save the picture with a new name so as not to replace the original. Look the picture up in windows explorer and right click on it, then choose properties and it will tell you the size of the picture. As long as it is 124kb or smaller, you will be able to upload it. Sounds like a lot of instructions, but after you do a couple it is really easy.
There are other ways to do it, especially if you have photo editing software, but just about everybody has Paint.