It is a great idea to add your company logo to your signature as it communicates legitimacy to anyone who receives it and makes your postcards look professional. 

To add your company logo, head over to Mail.

Click Postcards.
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Click the three dots at the lower right corner of a template.

Click Edit Design.

Click Signature.

Scroll down and look for Signature Image.

Click Upload Photo.

Select a picture from your computer.

Click Return to Layers.
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Click Add New Layer.
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Click Image and under Image Type, select "Use Signature Image".
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Move the logo to your desired space (make sure that the "Advanced Editing Mode" at the bottom is enabled to be able to move the layer for the logo.

Click Save Design.