\documentclass{article}
\usepackage[margin=8mm]{geometry}
\usepackage[object=vectorian]{pgfornament}
\usepackage{pgfplots}

\usepgfplotslibrary{external} % creates a tight self-contained pdf figure for each tikzpicture
\tikzexternalize % comment out to debug if latex errors: generates the external pdf
\tikzset{external/force remake} % force remarking of external pdf otherwise will use external pdf

\tikzsetexternalprefix{figures/} % output the pdf to an existing directory (needs to exist)
\tikzsetnextfilename{pgf-ornament}

\tikzset{png export/.style={
	external/system call={
		pdflatex \tikzexternalcheckshellescape -halt-on-error -interaction=batchmode -jobname "\image" "\texsource";
		echo "\\n---- Checking convert available ----\\n\\nPATH:" $PATH "\\n";
		which convert; echo; convert -version;
		convert -units pixelsperinch -density 150 "\image.pdf" "\image.png";
		convert -units pixelsperinch -density 600 "\image.pdf" "\image-600.png";
	}}}

\tikzset{png export}
\pgfplotsset{compat=1.12}

\begin{document}
\pgfornament{88}
\end{document}
