I find that the best sort of image to work with is obviously one made up of lots of lines, some of the best are those nice little painting illustrations that are always part of the instruction sheet for model kits.
If you're using one of these, set your scanner to text mode, 200dpi seems to work best and scan. Then clean up the image in DPaint, this is the key step, so don't skimp, get an image made up of smooth sharp lines.
If you are using a colour image, you need to get it into two colours, for this either start drawig on top in DPaint, then stencil your new lines and rub out the image. One method that can work, is to use an Emboss command (have a look at PBM tools), this will effectivly outline different blocks of colour, you can then just touch it up in DPaint. Beware that you tend to loose a lot of dimensional accuracy doing this unless you have a really hires image, so be careful.
If the drawing is to scale, mark a scale on it, a simple graduated bar, preferably one horizontal and one vertical round the part of the image that you are interested in. When you clean up the drawing make sure these scale marks are clear. You can later use these marks to scale the bitmap to an imagine grid.
Now use ConvertIFF/ILBM, say no to the add faces requester, and in a minute or so all the lines will appear. Now all you have to do is to go into pick points mode and tidy it up a little more. Save this.
If you have a plan, front and side view you can position these objects so they are visible in their respective window, but not visible in the other windows, i.e position the front view so it is say, 500 units in front of the world-centre etc. then redraw speeds are quite nippy.
Using this method may be a bit more memory intensive than using a bitmap, but you can zoom in to see finer detail. Also the images can be to a known scale and you can scroll the windows about without loosing the image.