Try something like e.g.
file:///C:/Bitmaps/image.jpg>r70%
or
[file:///C:/Bitmaps/image.jpg>r70% ] (space before ']' is important)
This scales "image.jpg" down to 70% of its original width and height. If
you want a non-proportional scaling, you can also use e.g.
file:///C:/Bitmaps/image.jpg>r70%x50%
for 70% of width and 50% of height.
Michael