>Looking at the GDIPlusX methods and writing code is out of the question ;)

Why not ???

The codes are really very simple to be understood.
Just see the codes that are currently resizing the image, and just instead of applying a specific value for the new size, apply the following formula:

lnNewWidth = lnOriginalWidth * lnRatio

Where
lnRatio > 0; is the percentage to be applied to the image

That's all you have to know :-)