This is the current version of my undergrad conference poster. It must have a header at its bottom, such as in the sample .ppt:

• Welcome to TeX.SX! Please help us help you and add a minimal working example (MWE) that illustrates your problem. Reproducing the problem and finding out what the issue is will be much easier when we see compilable code, starting with \documentclass{...} and ending with \end{document}. Please do not link to external information as this may be lost. You can add both your source code and an image result in your question – BambOo Oct 12 '18 at 13:57
• Thanks for the greetings. Since the question is about formatting, I can't think of how to build a minimal working example here. The overleaf link has the code I'm using, but if I pasted it here it'd be way too long. How should I proceed? – big-lion Oct 12 '18 at 14:26
• You should provide a light example (hence minimal) sufficient to reproduce you problem but without unnecessary piece of code – BambOo Oct 12 '18 at 14:38
• You could replace the \color{imeRed}{\rule{\textwidth}{0.2cm}} (which is wrong syntax, \color{...} is a switch and only takes one argument) with an image of the footline. – user36296 Oct 12 '18 at 16:50
• Off-topic: never use .jpg for graphs, the quality is horrible. Please also reconsider the choice of font. I would find it difficult to read on a poster. – user36296 Oct 12 '18 at 16:58

Welcome to TeX.SE.

You can add the image using the fancyhdr package as a page footer. This answer places it as a footer in relation to the page coordinate, current page.south. This is done by creating a node into which you insert your image sic. The anchor of the image is its south coordinate. The result is that the south coordinate of the page is connected to the south coordinate of the image. The width of the image is equal to \pagewidth, which was passed to geometry and set in the class options for a0poster (declaring it in geometry is probably redundant).

You also need to load the graphicx package to use \includegraphics to add the image. The poster body is filled with text from the lipsum package.

Help will always come much faster when you supply a minimum working example that people can provide input to. Just setup the example and explain what you are having trouble with. Happy TeXing.

This is a MWE:

\documentclass[a0,portrait]{a0poster}

\usepackage{geometry}
\geometry{paperwidth=60cm,paperheight=90cm,margin=4cm}

\usepackage[T1]{fontenc}       % output de ácentos
\usepackage[utf8]{inputenc}    % input de àcentos
\usepackage{lipsum}      % <--- Added for blind text
\usepackage{graphicx}    % <--- Added to insert footer image
\usepackage{tikz}
\usepackage{multicol}  % multi colunas
%--------------------------------------
\renewcommand{\footrulewidth}{0.0pt}
\fancyfoot{
\begin{tikzpicture}[overlay,remember picture]
% a generic image would be example-image-plain
% the image "sic" was supplied by OP
\node (footer) at ([yshift=-0.2cm]current page.south) [anchor=south] {\includegraphics[width=\paperwidth,height=4cm]{sic}};
\end{tikzpicture}
}
%------------------------------------

\begin{document}

\pagestyle{fancy}
\begin{minipage}{0.95\textwidth}
\begin{center}
{\color{blue} \Huge \textsc{Title}}
\end{center}
\end{minipage}

\hrule

\setlength{\columnsep}{40pt}

\begin{multicols}{2}
\lipsum[1-16]
\end{multicols}

\end{document}


This is the result: