I modified a few snippets from Tom from http://texblog.org/ to suit my needs. The structured LoT made by the original doesn't work with longtable. Also, it structures by part, not chapter, but if you need chapters, that's easy to change by following to the original: http://texblog.org/2012/01/18/adding-additional-structure-to-the-list-of-figures…

this.is.not.the.best.file.in.the.world.this.is.just.a.snippet.tex

% Lists of figures, tables and listings with part headings
\usepackage{etoolbox}

\makeatletter
% Create a flag
\providebool{newpartfig}
\providebool{newparttab}
\providebool{infigure}
\providebool{intable}
% Fix the \part command to set a flag
\newcommand{\saved@part}{}
\let\saved@part\part
\renewcommand{\part}{%
  \@ifstar {\saved@part*}{\@dblarg\my@part}}
\newcommand*{\my@part}[2][]{%
  \saved@part[#1]{#2}%
  \global\setbool{newparttab}{true}%
  \global\setbool{newpartfig}{true}}
% Fix the \caption command to check the flag and insert a LOF line
\let\LT@oldmakecaption\LT@makecaption
\renewcommand{\LT@makecaption}[3]{%
\LT@oldmakecaption{#1}{#2}{#3}%
\ifbool{intable}{%
\ifbool{newparttab}{%
    \addtocontents{lot}{\protect\contentsline{part}{%
    \vskip0pt \hskip20pt Part \thepart \vspace{3pt}}{\protect\relax}{}}%
    \global\boolfalse{newparttab}%
}{}%
}{}%
}
\let\oldcaption\caption
\renewcommand{\caption}[2][\shortcaption]{%
\def\shortcaption{#2}%
\ifbool{intable}{%
\ifbool{newparttab}{%
    \addtocontents{lot}{\protect\contentsline{part}{%
    \vskip0pt \hskip20pt Part \thepart \vspace{3pt}}{\protect\relax}{}}
    \global\boolfalse{newparttab}
}{}%
}{}%
\ifbool{infigure}{%
\ifbool{newpartfig}{%
    \addtocontents{lof}{\protect\contentsline{part}{%
    \vskip0pt \hskip20pt Part \thepart \vspace{3pt}}{\protect\relax}{}}%
    \global\boolfalse{newpartfig}%
}{}%
}{}%
    \oldcaption[#1]{#2}%
}
% Add in figure/table detection
\AtBeginEnvironment{figure}{
    \global\booltrue{infigure}%
    \global\boolfalse{intable}%
}
\AtBeginEnvironment{table}{
    \global\booltrue{intable}%
    \global\boolfalse{infigure}%
}
\AtBeginEnvironment{longtable}{
    \global\booltrue{intable}%
    \global\boolfalse{infigure}%
}
\makeatother