Custom alignment of columns in align environment

Making align break across pages is simple: add \allowdisplaybreaks[1] to your preamble.

And since your first column is text, it's also trivial to center it within align: just wrap it in a \makebox larger than all the images (which centers its contents) and use align as normal.

\documentclass{article}
\usepackage{amsmath}

\allowdisplaybreaks[1]
\newcommand{\centerdia}[1]{\makebox[2in]{\includegraphics{#1}}}
\begin{document}
\begin{align}
  \centerdia{dia1} & c=20x^2+5x-10+\frac{x^3-4x^2+500x-f(240)}{50}  \\
  \centerdia{dia2} & \mu=10*\epsilon\\
  \centerdia{dia3} & \mu=10*\epsilon
\end{align}
\end{document}

Use a modified version of align; change 2\tabcolsep into the separation you prefer.

\documentclass{article}
\usepackage{amsmath,environ}
\makeatletter
\NewEnviron{specialalign}
  {\def\align@preamble{%
     &\hfil
      \strut@
      \setboxz@h{\@lign$\m@th\displaystyle{####}$}%
      \ifmeasuring@\savefieldlength@\fi
      \set@field
      \hfil
      \tabskip2\tabcolsep
     &\setboxz@h{\@lign$\m@th\displaystyle{{}####}$}%
      \ifmeasuring@\savefieldlength@\fi
      \set@field
      \hfil
      \tabskip\alignsep@
  }%
  \begin{align}\BODY\end{align}}
\makeatother

\begin{document}
\begin{specialalign}
x & a=b \\
yyy & c=d+e+f \\
zzzzzz & 1\ne0
\end{specialalign}
\end{document}