You'll have to do some math, unfortunately. The general process to scale an image to a specific altitude range looks like:

1) Decide what the highest and lowest values in your input image represent. Let's assume that the lowest grayscale level is altitude 100 and the highest grayscale level is altitude 25000.
2) Load the image into Wilbur as a grayscale image (File>>Open, type as Grayscale Image Surface).
3) Use Filter>>Mathematical>>Span. Set Low to your lowest desired value and High to your Highest desired value. Click OK.
4) Look at the image. It is now scaled to the desired altitude range.