Aside from all the legal issues mentioned here is how I do it:
draw up your seal with cirles and text in a circular pattern as needed.
for the signature - use the sketch command until you get a pretty decent look.
This way there are no images to load.
Save all as a block
In answer to your question:
Keep the stamp & signature independent. Have the stamp i CAD format, referenced into your title block file.
If you are skilled, you could do the same thing with your signature block. Scan a dignature into ,df, and bring it into cad, then trace it using the sketch (poly,spline,etc.) command. A vector file doesn't need an mage frame when it is referenced into another file.
Regrding other responses:
IF SOMEONE WANTED TO STEAL A STAMP, IT WILL BE STOLEN. Todays media software is fantastic. The technogy is out there for convenience, or for mal-intent, but to think that your seal is safe because you save your files a certain way is naive. The single best way of covering you liability exposure is to keep an independent stamp log of what was officially submitted. In the court of Law, this would be admissable evidence, provided you are diligent in keeping it current.
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