Scale the image to three quarters of its original size as shown below.
Code: Select all
\documentclass[11pt,a4paper,spanish]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage[font=small,labelfont=bf,tableposition=top]{caption}
\usepackage{graphicx}
\begin{document}
\begin{figure}[!ht]
\centering
\includegraphics[scale=0.75]{EJ1SIS}
\caption{Esquema de transmisión de datos con un cifrador-aleatorizador}
\label{fig:ej1sis}
\end{figure}
\end{document}
The reason for that is the relation between
postscript point and
pixel. See forum search for details.
As an alternative you can draw this picture with
pgf/tikZ or
PSTricks.
If this solves the issue, please mark the topic as solved by editing the initial post and choosing the green checkmark from the icon list.
Best regards
Thorsten