Starting with an image on your computer. Open "my computer" or "Windows explorer"
Right click on the picture file it will read "open with" then highlight that and choose "Paint."
In Paint, choose "image". In the drop down menu pick "stretch/skew"
The image size will pop up. Change the value to about 33%. That works for me and my picture size, you may have to adjust for yours.
You will need to change the percentage values ONLY in the Stretch section and keep them equal, meaning if you change the 1st value to 33 then the 2nd value has to be 33 too, to keep it proportional.
Then all you need to do is save it.
You may have to do this more than once to get the correct size you need, but once you figure the correct values then it should be the same for all the rest.
Here is a pic of the screen you need:

Also if you have your camera set at its lowest picture setting that helps too. Mine won't take a pic less than 1024x786 so I set it there and take the pic quality setting down to the lowest setting too. Once the pic is taken, i still have to go into paint and resize. Sometimes i take the values down 50% and others down only 20% of the origanal pic size. so once you find out what size you have to start with, start playing from there.