Left-justify text in LaTeX

I recommend you load the ragged2e package (part of the ms suite of packages) with the document option:

\usepackage[document]{ragged2e}

With this setup, all parts of the document will be typeset left-justified (or "flush-left") rather than fully-justified. The term "all parts of the document" includes:

• regular material in the body of the document -- mostly, all paragraphs
• all footnotes
• all material in minipage environments
• all p-type columns in tabular-like and array environments
• all material in \parboxes
• all material in \marginpar directives.

In contrast, inserting the "standard LaTeX" \raggedright directive immediately after \begin{document} affects only the "regular" material; it does not affect any of other five types of material, at least not when executed solely after \begin{document}.

Relative to the \raggedright directive, employing the ragged2e package also preserves TeX's ability to hyphenate words. This, in turn, avoids creating the excessive line raggedness that almost invariably results from using \raggedright.

A separate observation: In the standard LaTeX document classes, material contained in a "floating" environment -- such as figure and table -- is aligned flush-left by default. If you want to center the contents of these environments, you may do so by issuing a \centering instruction after the \begin{figure} and \begin{table} directives. (The scope of the \centering instruction ends automatically when LaTeX processes the \end{figure} and \end{table} statements.)

\documentclass{article}
\usepackage[document]{ragged2e}
\usepackage{lipsum,xcolor}
\begin{document}
%%\raggedright % this directive only affects "regular" material

\marginpar{\textcolor{red}{Marginpar}\\\lipsum[2]}
{\color{red}Regular Material}\\
\lipsum[2]

aaa\footnote{\textcolor{red}{Footnote}\lipsum*[2]}

\textcolor{red}{Minipage}\\
\begin{minipage}{\textwidth}
\lipsum[2]
\end{minipage}

\textcolor{red}{Tabular}\\
\begin{tabular}{@{}p{\textwidth}@{}}
\lipsum[2]
\end{tabular}

\textcolor{red}{Parbox}\\
\parbox{\textwidth}{\lipsum[2]}
\end{document}

As far as TeX's alignment goes a graphic is just the same as a big letter, it does not treat it any differently.

If you want your entire document flush left it is better to use the declaration form \raggedright after \begin{document} and then use \begin{center} or \begin{flushright} around your \includegraphics. Of if they are already in an environment such as figure perhaps use the declaration forms \centering or \raggedleft rather than the environments.