Hi Akesson,
You need your decal in a format that is easy to work with in the drawing. What it sounds like you need is to make a drawing symbol showing your decal.
You COULD insert the scan of the decal as an image and place it in your drawing as an object, but that is likely going to create more problems than it will solve. Having the file stored as a vector image and as a symbol ensures that you can reuse this data over and over both on drawings and on models.
If you already have the decal in a CAD format (iges or dxf, for example) or even as a vector image (many print shops use *.ai files from Illustrator) then half your work is done.
Under Format ==> Symbol gallery in drawing mode, you can create a symbol by importing the decal using Insert ==> Shared Data->From file.
If you don't have the file as Vectors (like a JPG or BMP), you should convert it. Illustrator has limited capabilities in this respect. Streamline is probably the best solution, but buying it and learning it may not be worthwhile for your application. Look up your local graphic design firms and see who can create vector files from your images. We've used these services to scan in old engineering drawings and then build 3D models from the vector files, and the quality was great.
Hope this helps..
All the best
Mark