Using a picture as signature

A user has saved a signature in their profile by uploading an image (.png & .jpg).
If he wants to sign a document that was sent to him via AdobeSign, this signature is not displayed to him, although it is saved in the profile. 
If he adds a signature to his profile using the mobile phone option, the option will be displayed to him immediately when signing a document that was sent via AdobeSign to him.

Why does this work with the mobile option but not with the picture option?