Poster in LaTeX

This tutorial sums up some possibilities how to prepare various size posters in LaTeX

http://www.eng.auburn.edu/~reevesj/Classes/ELEC6970-latex/posters/BEAMERPOSTER.pdf

How to force no indentation at the beginning of a new section

Use the \noindent command at the beginning of the section. This will force the first line to have no indentation at all.

Include figures from a separate directory

It is often a good idea to have the figures separated into a folder. This can be done using the \graphicspath command as

\graphicspath{{./mypics/}}

after the includes.
This will tell the compiler to look up the figures from the mypics directory in the document root. Don't forget to include the graphicx (or similar) package.