Let each individual user upload a custom picture for their signature.

Thanks for sharing! In Trengo we have a few tricks to make the signature a lot more personal. 

1. You can add an agent's profile picture to the signature, using the tag [agent.image]. 
2. An agent can have custom fields in which you can fill in personal information. 
   You can define the subject of this personal field in any way you want (like working days, ore job description for example. Later, you can use [tags] in the signature to refer to these fields (agent.custom_field.[customfieldid])

Please let me know if this is what you were looking for, or whether you wish for more personalization.