\documentclass[a4paper,12pt]{article}
\usepackage{amssymb,amsmath} % needed for math
\usepackage{standalone} % needed for includestandalone
\usepackage[utf8]{inputenc} % this is needed for umlauts
\usepackage[ngerman]{babel} % this is needed for umlauts
\usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf

\usepackage{adjustbox}
\usepackage{xcolor}
\definecolor{myLightGray}{HTML}{F9F9F9}
\definecolor{borderColor}{HTML}{CCCCCC}

\usepackage{tikz} % needed for includestandalone
\usetikzlibrary{arrows,positioning, calc}

\tikzset{
    pil/.style={->,thick}
}

\newenvironment{gallery}{%
    \center
}{%
    \par\endcenter
}

\def\galleryimage#1{%
    \adjustbox{width=4cm,height=4cm,keepaspectratio, center=4cm,
    valign=M, set vsize={2cm}{2cm},
    %or: raise={.5\depth-.5\height}{2cm}{2cm},
    bgcolor=myLightGray,cfbox=borderColor 1px 0px 2px}
    {\includestandalone{#1}}%
    \space\ignorespaces
}%

\begin{document}
Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam 
nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam 
erat, 
sed diam voluptua. At vero eos et accusam et justo duo dolores et ea 
rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem 
ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur 
sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore 
et dolore magna aliquyam erat, sed diam voluptua. At vero eos et 
accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, 
no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum 
dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod 
tempor invidunt ut labore et dolore magna aliquyam erat, sed diam 
voluptua. At vero eos et accusam et justo duo dolores et ea rebum. 
Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum 
dolor sit amet.   

\begin{gallery}
    \galleryimage{dot-product-1}
    \galleryimage{dot-product-2}
    \galleryimage{dot-product-3-1}
    \galleryimage{dot-product-6}
    \galleryimage{equilateral-triangle}
    \galleryimage{equilateral-triangle-heights}
\end{gallery}


Duis autem vel eum iriure dolor in hendrerit in vulputate velit esse 
molestie consequat, vel illum dolore eu feugiat nulla facilisis at 
vero eros et accumsan et iusto odio dignissim qui blandit praesent 
luptatum zzril delenit augue duis dolore te feugait nulla facilisi. 
Lorem ipsum dolor sit amet, consectetuer adipiscing elit, sed diam 
nonummy nibh euismod tincidunt ut laoreet dolore magna aliquam erat 
volutpat.   

\end{document}
