Perhaps the multienum package could be an option:
\documentclass[12pt]{article}
\usepackage{amssymb} %for \blacksquare
\usepackage{multienum}
\newcommand{\rsqr}{\raisebox{0.4ex}{\tiny $\blacksquare$}}
\newlength\myindent
\newlength\mylen
\renewcommand{\itemxx}[2]{%
\setlength\mylen{\remainxx}
\addtolength\mylen{-\myindent}
\hskip\myindent
\parbox[t]{\labelwidth}{\hfill\labelenumi}%
\hskip\labelsep
\parbox[t]{0.5\mylen}{\raggedright #1}%
\hfill\parbox[t]{\labelwidth}{\hfill\labelenumi}%
\hskip\labelsep
\parbox[t]{0.5\mylen}{\raggedright #2}\smallskip}
\newenvironment{listable}[1][0cm]
{\begin{multienumerate}
\setlength\myindent{#1}
\renewcommand\labelenumi{\rsqr}
\setlength\itemsep{-.5\baselineskip}
}
{\end{multienumerate}}
\begin{document}
\begin{listable}
\mitemxx{Lots and lots of text, oh my!}{More and more text}
\mitemxx{word}{All work and no play makes Jack a dull boy}
\end{listable}
\begin{listable}[4em]
\mitemxx{Lots and lots of text, oh my!}{More and more text}
\mitemxx{word}{All work and no play makes Jack a dull boy}
\end{listable}
\end{document}
The optional argument of the listable
environment controls the indentation from the left margin (default value: 0cm).

All problems are not resolved. You need to create a macro to automate all the little jobs and I have not used Arial and 8pt.
It will be fine to cretae a macro with argument the caption and the picture. I placed a minipage
inside a minipage
to get margins around the caption. I think there is perhaps a better way to this. I used a box to get the height of the picture, I don't know if it's possible to avoid this.
update 4: I added a macro like Herbert and I adapted the code to odd and even pages but you need to make some calculations to replace \hspace*{-6em}
and \hspace*{-4em}
by something like \hspace*{\dimexpr -\marginparwidth-\marginparsep\relax}
(Herbert's code). I don't know exactly what are the macros for a twoside
document to calculate the left and right margin.
I corrected 2 bugs. I added some extra space around the picture
\documentclass[11pt]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier,xcolor,graphicx}
\usepackage[font={sf}]{caption}
\usepackage{ifthen,changepage,lipsum}
\newsavebox{\mybox}
\DeclareCaptionLabelSeparator{period-newline}{.\newline\newline}
\captionsetup{aboveskip=3pt,singlelinecheck=false,
labelsep=period-newline,labelfont={small,bf}}
\newcommand\Image[4][width=\textwidth]{%
\savebox{\mybox}{\includegraphics[#1]{#2}}
\fboxsep=0pt
\checkoddpage
\ifthenelse{\boolean{oddpage}}{%
\hspace*{-25mm}
\hbox{\colorbox{black!10}{\rule{0pt}{\dimexpr \ht\mybox+4mm}%
\begin{minipage}[b]{38mm}\center
\begin{minipage}[b]{34mm}
\caption[courte]{#3}
\label{#4}
\end{minipage}%
\end{minipage}}%
\hspace*{2mm}
\colorbox{black!25}{%
\begin{minipage}[b]{145mm}\center
\vspace*{2mm}
\colorbox{white}{\usebox\mybox}%
\vspace*{2mm}
\end{minipage}}}%
}{%
\hspace*{-40mm}%
\hbox{\colorbox{black!25}{%
\begin{minipage}[b]{145mm}\center
\vspace*{2mm}
\colorbox{white}{\usebox\mybox}%
\vspace*{2mm}
\end{minipage}}%
\hspace*{2mm}
\colorbox{black!10}{\rule{0pt}{\dimexpr \ht\mybox+4mm}%
\begin{minipage}[b]{38mm}\center
\begin{minipage}[b]{34mm}
\caption[courte]{#3}
\label{#4}
\end{minipage}%
\end{minipage}}}%
}
}
\begin{document}
\section {Figures/Capturing}
Place Tables/Figures/lmages in text as close to the reterence as possible. (see Figure 1). The table, frgure or image has to put in the area which is width 14.5 cm., filled with 70\% black colm. You should leave 2 mm. tram margin. In a side oox (width 3.8 cm., filled with 15\% Black), a short caption could be in the bottom.
\begin{figure}[h!]
\Image[width=8cm]{elephant.pdf}{Long texte de légende avec quelques explications sur une figure importante}{elephant}
\end{figure}
ln tables, pictures, images or captures. use 10pt Arial regular to describe. Each f<gure (group) must include a caption set in 8-points Arial regular. The caption is to be on left or right depends on odd or even page. Figure numbering and referencing should be done sequentially, e.g. Figure. 1, Figure. 2, Table 1., Table 2 .. etc. for single figure and Figure l(a), Figure 1(b)., etc., for figures with multiple parts.
\lipsum [1]
\section {Figures/Capturing}
\begin{figure}[h!]
\Image[width=8cm]{elephant.pdf}{Long texte de légende avec quelques explications sur une figure importante}{elephant bis}
\end{figure}
Compare with the first picture \ref{elephant}
\end{document}

Best Answer
Floats are only marginally tied to the place where they appear in the LaTeX document; that place basically means "starting from here, try and find a suitable spot where the float can be typeset". The caption and the number are in fact thought for giving a description and defining a label for referencing the object.
According to Murphy's law “the place where I'd like the float to appear is never the one that will be eventually chosen”; this seems to be a joke, but it's really what happens in the majority of cases: particularly, if a float is large, the chances it can be placed at the “right spot” are small.
I'd add also that I find it dubious breaking an enumerated (or itemized) list with a float, because it hinders legibility more than if the float is at the top or bottom of the page with an appropriate reference in the text.
In general, LaTeX is not more reluctant to place floats in the middle of a list than in other places. But it respects the relevant parameters' values. A good place to look for information is Frank Mittelbach's answer to How to influence the position of float environments like figure and table in LaTeX?