You shouldn't need to do anything to the image file itself. It will be scaled at the time of printing (or inserting into another document if that's what you're doing), and it will simply end up at whatever dpi it scales to.
The image is defined as its pixel size, so 22x17 at 150dpi is 3300x2550 pixels. Dots per inch (DPI) is only used when printing or displaying on the screen. But 22x17 is a different aspect ratio than 8x6, so if you want 8" wide it will be 412.5dpi but it would be 6.18" high so some of the picture would be cropped, or if you want it 6" high it will be 425dip only 7.76" wide.