Lets' say I want to emulate an underlined signature; so I have this image:

... which has been generated with the ImageMagick convert line included in this MWE:
\documentclass{article}
\usepackage{graphicx}
% create testing.png using ImageMagick convert
% call w/ `pdflatex -shell-escape test.tex` to have the command run
% note: \includegraphics ignores the 150dpi set here, and will
% output bigger image than 2x0.6 in; *unless* -units pixelsperinch is set
% - but then it may give wrong resoltion in `identify -v`?...
\immediate\write18{convert -units pixelsperinch -density 150 -size 300x100 xc:white -draw "decorate UnderLine font Comic-Sans-MS-Regular font-size 30 fill dodgerblue text 20,70 ' Testing '" testing.png}
\begin{document}
Just testing something here... Lorem ipsum dolor sit amet,...
\noindent%
Attempt at signature: \makebox[0pt][l]{\hspace*{2cm}\vspace*{-2cm}\includegraphics{testing.png}}\hrulefill
That's all, folks ...
\end{document}
The document compiles to this:

I was basically trying to output the image first as a \llap (its \mbox construct [I guess] is taken from \llap (or \rlap) at the beginning of an indented paragraph), and then have the horizontal under-line via \hrulefill typeset over the image. That generally seems to work.
Then I was trying to position the image, so that its underline aligns with the \hrulefill. As How to shift graphics/adjust placement of figure with \includegraphics notes, \hspace* works here (\hfil unfortunately doesn't work, as the overlap \mbox apparently has to have width of 0pt) - and I'm pleased with the horizontal position.
The problem is the vertical position - how can I push the image "down", so its underline aligns with the \hrulefill one? Clearly \vspace* doesn't work (and using something like \rule{0pt}{2cm} moves all of the rows, while the relative vertical position between the image underline and the \hrulefill one does not change).

\raisebox? – egreg May 09 '15 at 22:54:)will try it now... – sdaau May 09 '15 at 22:54