Not entirely certain, but I think you are trying to put an image on a 1km basemap. If so the best way to get the right size image is to forget about the eye altitude in GE. Use the GE rulers to draw a line 1,000 meters long and capture the image. In your graphics editing program set up a square cropping tool exactly the size of the ruler line and crop 1km squares that way. Re-size the cropped images to 1024 x 1024 and job is done....