Adobe Reader DC -Graphic for Signature - only PDF-Files possible

Hi 

I would like to setup a digital signature in adobe reader.

Preferences>Signatures>Creation and Appearance Preferences > New> Configure Graphic > imported graphic > File => there i can only select pdf-files to import. I need to import a png file with a signature image.

This occures only in Adobe Reader. In Adobe Acrobat Pro is it possible to select other files like png or jpg.

Is this a restriction of Adobe Reader? Or what i have to do?

Best regards

Simon