Every user account has a Personal Profile (accessed by clicking on the icon under "Settings>Personal Profile" or under "Settings" in the account header). Within the personal profile settings there is a section to create three different Signatures - a "Basic Signature," a "Signature with Photo," and a "Signature with Logo" - and the ability to chose which of these (or none) to be their default signature when creating a new email to a lead.
Click on any of the sections to add or edit your signature. By default you can use the text from your basic signature for each of the additional signatures you create, or you can create completely unique ones for each.
The "Photo Signature" can use your profile pic, or you can choose to upload a different one.
The logo signature allows for a wider logo image (if needed).
Users also have the option to chose which signature - or to use none at all - as the default when creating a new email message to a single lead in Open Leads. Just above the Basic Signature is a line to choose the default address.
Mousing over and clicking on this line gives the user a dropdown to choose their default signature.
When this is chosen, any single emails composed and sent from the lead detail screen will contain this signature. Should "none" be chosen as the default, any of your signatures can be inserted in your message using a Token.
Using Tokens in Emails
NOTE: Default signatures do not apply to mass email messages