The way DesignCAD imports images is rather haphazard. Images are imported at the screen resolution of the particular computer. Each image pixel matches a screen pixel, and this bears no relationship to drawing units.
If you import an image that is 1000x1000 image pixels, it will be placed in the drawing at 1000x1000 screen pixels. Then if you zoom out 2X and import the same image, it will again be imported at 1000x1000 screen pixels. But any existing drawing elements will be only 1/2 as wide on the screen as they were before the zoom, and the second copy of the image will be twice as wide relative to other drawing elements, including the first imported copy of the image.
This makes it very difficult to import multiple scanned images of parts of the same paper drawing at the same relative size. If you change the zoom levels between image imports you are screwed!
This is pretty lame!
Most (all) image files contain horizontal and vertical resolution information, expressed as dpi (dots per inch) - or some other unit of measurement.
If a drawing has defined measurement units the user should have an option to import the images at the dpi resolution included in the image. If drawing units have not been assigned (a "dimensionless" drawing) import the images just bring the images in at the resolution in the image relative to drawing units.
You could also allow the user to set a scale multiplier for imported images. Setting a scale multiplier of 10 would import a 300 dpi image at 3000 dpi, and a scale multiplier of 0.1 would import the 300 dpi image at 30 dpi. The number of pixels in the image would not be changed - only the way the image is displayed in drawing units would change.
The real virtue of using the image dpi for imports is that multiple images would import at the same scale relative to the drawing units. This would allow importing multiple scanned images of parts of a large paper drawing at the same scale, Zooming in/out would not change the relative size of the image if the same image was imported multiple times.