I put the following in:
GdipBitmapSetResolution(img, 300, 300);
And now when I load the file in Mspaint I get a resolution showing 300 instead of 96 which was the case before: however: the file sizes are relatively unchanged between 96 and 300. If I use the scanner’s software to save the file I get a significantly larger file, and the resolution is much more distinctive.
Are there other settings that need to be changed?