Short answer
HubSpot's KB: the Simple editor can insert a link or image, or click HTML to edit the code. You can set a font with the face attribute, but many fonts aren't supported across email clients. Managing multiple signatures needs Sales Hub Professional or Enterprise and a Sales seat.
1. The editor
Signature settings in my demo portal.
2. Sharp images
In HTML, set the image width you want displayed so clients don't rescale it unpredictably.