Hello @TheJupiterOne and welcome to the forum 
Try these replies to a similar topic:
Thickness of stroke - #5 by AhabGreybeard
Thickness of stroke - #7 by djwaterman
(You don’t need to install G’MIC nowadays. It’s included in krita.)
If they don’t help, explain why they don’t help and upload a full screen screenshot of a signature image opened in krita to give people a better idea of the nature of the problem.
Edit:Add: I’ve just remembered, there’s the Stroke layer style that can be used with lines on a transparent background and can turn this:
into this:
So, what is your initial situation with the signature images?
What do they look like?