One among many possible solutions is the use of eso-pic to draw your style. Notice you may editing the measures or make them automatic to any page geometry your are using.
This solution applies the same background to each page. If your are looking for a header in specific page, I may check how to adapt it using fancyhdr for specific cases. I think you would like it since you wrote fancyhdr and it seems to be a page in a report.
OBS.: As you are writing in Brazilian Portuguese, I suggest using \usepackage[brazil]{babel} and \usepackage[utf8]{inputenc} (it'd be probably better).
\documentclass[letterpaper,12pt,oneside]{book}%
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage{geometry}[showframe]
\usepackage[svgnames]{xcolor}
\usepackage[brazil]{babel}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{eso-pic}
\usepackage{lipsum}
\AddToShipoutPictureBG{%
\begin{tikzpicture}[remember picture, overlay]
\fill[opacity=1,line width=2pt,DarkGreen]
($ (current page.north west) + (0cm,-2cm) $)
rectangle
($ (current page.north east)$);
\node[opacity=.8, inner sep=0pt] at ($ (current page.north west) + (3cm,-1cm) $)
{\includegraphics[height=1.5cm]{example-image-duck}}; %*(\thepage-1)
\node[font=\bfseries\LARGE, white] at ($ (current page.north) + (0cm,-1cm) $) {Dicionário de Dados};
\end{tikzpicture}
}
\begin{document}
\lipsum[1-10]
\end{document}

Edit:
Adapting the previous example for fancyhdr, you can add your header to the pages you want and only them.
I'd suggest to learn how to Make your own .sty files and making this style part of your own library of styles.
\documentclass[letterpaper,12pt,oneside]{book}%
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage{geometry}[showframe]
\usepackage[svgnames]{xcolor}
\usepackage[brazil]{babel}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{eso-pic}
\usepackage{lipsum}
\usepackage{fancyhdr}
%\pagestyle{mystyle} % fancy, myheadings, headings
\fancypagestyle{mystyle}{ % alteração de estilo pré-definido.
\fancyhf{} % clear all header and footer fields
\fancyhead[R]{\thepage} % except the right top corner
\fancyhead[L]{
\begin{tikzpicture}[remember picture, overlay]
\fill[opacity=1,line width=2pt,DarkGreen]
($ (current page.north west) + (0cm,-2cm) $)
rectangle
($ (current page.north east)$);
\node[opacity=.8, inner sep=0pt] at ($ (current page.north west) + (3cm,-1cm) $)
{\includegraphics[height=1.5cm]{example-image-duck}}; %*(\thepage-1)
\node[font=\bfseries\LARGE, white] at ($ (current page.north) + (0cm,-1cm) $) {Dicionário de Dados};
\end{tikzpicture}
} % except the left top corner
\renewcommand{\headrulewidth}{0pt} % remove line between header and main text
}
\begin{document}
\chapter{C}
\section{S1}
\pagestyle{headings}
\lipsum[1-10]
\clearpage
\section{S2}
\pagestyle{mystyle}
\lipsum[1-10]
\clearpage
\section{S2}
\pagestyle{headings}
\lipsum[1-10]
\end{document}
The following picture presents pages 4 and 5, the ones under \pagestyle{mystyle}. Pages from other sections don't have it.

Edit 2:
Using \fontfamily{phv}\selectfont at \node[font=\bfseries\LARGE, white] at ($ (current page.north) + (0cm,-1cm) $) {\fontfamily{phv}\selectfont Dicionário de Dados}; changes font to Helvetica, which is very similar to Arial and not easy to spot differences.
