I have used scanned or downloaded images for tracing purposes several times, with no problems. I have DesignCAD 3DMax 15, the latest downloadable version from IMSI.
I don't know how familiar you are with DesignCAD, so forgive me if I am telling you some things you already know.
Use File/Image/Load Image File. I have inserted JPEG, GIF, TIFF and EPS files successfully. You have the choice of inserting at full size (this is what you want, most likely) - simply doubleclick in the drawing window once the load dialog is complete, OR you can resize the image before pasting by clicking lower left/upper right corners for the pasted image. I don't think this latter function is very useful because it does not permit you to scale in a controlled fashion.
Although DesignCAD does provide an autotrace function for bitmaps, I have done all my tracing by hand because the autotrace is prone to picking up artifacts. You can tune the parameters for the autotrace but I could do a better job in less time by hand. Maybe if I had to do a lot of images it would be worth learning all the ins-and-outs of autotrace but...
You can set the drawing scale to whatever dimension you want after inserting the image. I simply measured the apparent distance (in DesignCAD internal units) from wingtip to wingtip, worked out the scaling factor to my desired span. After selecting the image, apply Edit/Selection Edit/Scale with the same x and y factors applied around the selection center.
I would recommend that you put the image into Layer 0, and LOCK that layer so it won't move. Then, do your tracing in a distinct colour in another layer. I didn't try to get the tracing bang-on, because the scanned images tend to be a bit pixellated and line widths can be a bit deceptive. I treated the tracing simply as a template, and LOCKED it into its own layer. I then began creating an accurate plan using many more layers above that.
USE LAYERS!!!
Good Luck
