Zooming in on an image

[Ben Fisher]

I'm wondering what is the most efficient way to resize a large image.
 I'm trying to create a "zoom-in" effect by displaying a 600x800 image and 
then slowly increasing the dimensions of the image (while still keeping the 
image centered on the screen). Ideally this would be smooth and non-cpu 
intensive.
 I wrote some code to do this. However, the results look awkward at best. At 
first, the zooming in is rapid, but then it slows down. After a while (when 
the zoom factor is about 4), the computer is bogged down and the image 
change is very slow and jerky. I would like to keep the rate even, although 
I will not know the processor speed of the client's machine. It is 
disheartening to me to see the performance so lousy on a Pentium 4 3.0 Ghz.
 Does anyone know a better way to "zoom in" images? It shouldn't be so 
processor-intensive - I tried "Preview" in WinXp and images easily zoom in 
and out without any lag.
 -Ben F