You don't need a tabular for that.
\documentclass[pdftex,10pt,letterpaper, oneside, article]{memoir}
\usepackage{amsmath}
\usepackage{tabularx}
\usepackage[pdftex]{graphicx}
\begin{document}
\begin{align}
n_u &= f_1 d_f t_1 \text{(N)}
&
\vcenter{\hbox{\includegraphics[width=8 cm]{./img/mode_a.jpg}}}
\end{align}
\end{document}
EDIT Since I was saying that it might look better with TikZ, I think I need to proof that.
\documentclass[10pt,letterpaper, oneside, article]{memoir}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{positioning,calc}
\begin{document}
\begin{align}
n_u &= f_1 d_f t_1 \text{(N)}
&
\begin{tikzpicture}[scale=0.7,font=\sffamily,font=\footnotesize,baseline=(c.base),
murmel/.style={rectangle,thick,ultra thick, minimum width=2cm,minimum
height=0.5cm, draw,inner sep=0pt,align=left,text width=1.9cm,
append after command={% courtesy of Alenanno ref: https://tex.stackexchange.com/questions/287967/drawing-thin-line-around-a-multipart-tikz-shape#comment696552_287972
\pgfextra{\draw (\tikzlastnode.north) -- (\tikzlastnode.south);}}}]
\node[murmel] (l1) at (0,0) {2};
\draw[-stealth,line width=1mm] (l1.west)-- ++ (-1,0);
\node[murmel,above=0cm of l1,xshift=0.5cm] (l2) {1};
\draw[-stealth,line width=1mm] (l2.east)-- ++ (1,0);
\node[murmel] (c) at (6.5,0.25) {1};
\draw[-stealth,line width=1mm] (c.east)-- ++ (1,0);
\node[murmel,above=0cm of c,xshift=-0.5cm] (r1) {1};
\node[murmel,below=0cm of c,xshift=-0.5cm] (r2) {2};
\draw[-stealth,line width=1mm] (r1.west) -- ++(-1,0);
\draw[-stealth,line width=1mm] (r2.west) -- ++(-1,0);
\draw[line width=1.8mm] ($(l2.north)+(0,0.1)$)--($(l2.south)+(0,-0.85)$);
\draw[line width=1.8mm] ($(c.north)+(0,0.85)$)--($(c.south)-(0,0.85)$);
\end{tikzpicture}
\end{align}
\end{document}
pdftex
option from the\documentclass
line and from\usepackage{graphicx}
. It does nothing and makes the document not portable.