Directories named in the TEXINPUTS environment variable are always
searched when an input file is named, so, for example, you could
set this variable as

setenv TEXINPUTS $HOME/Mytexfiles:

and keep your signature graphic in ~/Mytexfiles, and include it
with a simple

\includegraphics{signature.eps}

without path.

The trailing : shown in TEXINPUTS above is important.  It indicates that
all the usual default places should be searched too.  Without it,
TEXINPUTS would have to explicitly duplicate the full directory path
that the programs search by default, and that would be an invitation to
trouble down the road if an update changes that path.

A component of TEXINPUTS can end with // to indicate searching that
component and the entire subtree below it.