Digital signatures ARE certificate signatures, nothing else. Other things with pictures of a personal scribble (worthless) aren't digital signatures.
There are two types of signatures in PDF: electronic signatures, which are just images (stamps, text, image) and digital signatures.
If you want to use electronic signatures in Acrobat XI go to Fill&Sign->Place Signature. If a drag rectangle for the digital signature dialog comes up, you have selected digital signatures and Acrobat/Reader remembered it. Cancel it, go back to Fill&Sign->Place Signature and click a triangle to the left of "Place Signature". Then click on "Changed Saved Signature" and select the electronic signature type you want to use.
If you want to use digital signatures you can create custom digital signature appearance. Go to Edit->Preferences->Signatures->Creation&Appearance->More. In the "Appearances" section click "New". You will be presented with the dialog that allows you to create a custom appearance. If you want to put there your picture or image of your ink signature, you need to prepare this image as a file beforehand, select "Image" radio button, browse to the location of your image file. In Reader you can use only PDF as your image file. In Acrobat you can use many more file formats: JPEG, PNG, etc.