The problem is that \includegraphics
inside a tabular
leaves no space above the image, so it abuts the dividing line. Here, I use the \addvbuffer
macro of the verbatimbox
package to add a 3pt buffer above (and 0pt below) the image, and call that new form \Includegraphics
, with a cap I
.
\documentclass{report}
\usepackage{longtable}
\usepackage[demo]{graphicx}
\usepackage{verbatimbox}
\newcommand\Includegraphics[2][]{\addvbuffer[3pt 0pt]{\includegraphics[#1]{#2}}}
\begin{document}
\begin{center}
\begin{longtable}{|l|l|l|}
\caption{Molecular structures of the molecules chosen in the blind test}\label{big_tab} \\
\hline Molecular Structure & Category & Blind test \\ \hline
\endfirsthead
\multicolumn{3}{c}%
{{\bfseries \tablename\ \thetable{} -- continued from previous page}} \\
\hline Molecular Structure & Category & Blind test \\ \hline
\endhead
\hline \multicolumn{3}{|r|}{{Continued on next page}} \\ \hline
\endfoot
\hline \hline
\endlastfoot
\Includegraphics[width=1in]{I.png} & i & \multicolumn{ 1}{c|}{1st} \\ \cline{ 1- 2}
\Includegraphics[width=1in]{II.png} & ii & \multicolumn{ 1}{l|}{} \\ \cline{ 1- 2}
\Includegraphics[width=2in]{III.png} & iii & \multicolumn{ 1}{l|}{} \\ \hline
\Includegraphics[width=1in]{IV.png} & i & \multicolumn{ 1}{c|}{2nd} \\ \cline{ 1- 2}
\Includegraphics[width=1in]{V.png} & ii & \multicolumn{ 1}{l|}{} \\ \cline{ 1- 2}
\Includegraphics[width=2in]{VI.png} & iii & \multicolumn{ 1}{l|}{} \\ \hline
\end{longtable}
\end{center}
\end{document}

Don't engage in so much visual formatting. Instead, define some pertinent table parameters -- such as centering the column contents, the heights of the rows, etc -- beforehand, and then create a lean and reasonably easy to read table.
In the example below, I've created a dummy definition of your \VTGN
macro to make the code compilable. You'll notice a complete absence of \vspace{3mm}
and \cl
directives in the code. Observe the use of \bigstrut
to size the heights of the rows: it's an object with a depth of 4ex
below the text baseline and a height of 6ex
above the baseline. (The strut's total height is thus 10pt
.) Adjust these parameters as needed to get the desired spacing. The strut's width is 0pt
; hence it's not visible. One \bigstrut
per row suffices.

\documentclass{article}
\usepackage[paperheight=28cm, paperwidth=20cm, margin=2cm]{geometry}
\newcommand\VTGN[1]{#1} %% just so that this example compiles
\usepackage{tabularx}
\newcolumntype{Y}{>{\centering\arraybackslash}X}
\newcommand\bigstrut{\rule[-4ex]{0pt}{10ex}}
\begin{document}
\noindent
\begingroup
\setlength\tabcolsep{0.1pt} % default: 6pt
\begin{tabularx}{\textwidth}{ | *{7}{Y|} }
\hline
\bigstrut & addition & subtraction & multiplication & division & exponentiation & rootification \\
\hline
Vortigenu \bigstrut & \VTGN{+} & \VTGN{-} & \VTGN{*} & \VTGN{/} & \VTGN{\char"5E} & \VTGN{\char"40} \\
\hline
Earth \bigstrut & + & $-$ & $\times$ & $\div$ & ? & ? \\
\hline
\end{tabularx}
\endgroup
\end{document}
Addendum: I just noticed that you posted the following definition of the \VTGN
macro:
\newcommand{\VTGN}[1]{\setmainfont{Vortigenu}{#1}\setmainfont{Times New Roman}}
Using \setmainfont
in this manner is rather inefficient as well as quite complex; see Section 5 of the user guide of the fontspec package for a more in-depth explanation of this claim. I suggest you provide the following commands in the preamble:
\newfontfamily\vtgn{Vortigenu}
\newcommand\VTGN[1]{{\vtgn #1}} % Note the double curly braces
This definition of \VTGN
makes it unnecessary to execute \setmainfont
twice. Moreover, it works without having to know what the main document font happens to be.
Best Answer
Include the image in a
\fbox{}
and set the space around it. But use@{}
in tabular columns. See the MWE:Other solution could be make a minipage in each cell instead of a framed box (modify the lengths as needed):
But if you do not want to see the vertical lines in the table (ie,
{cc}
instead of{|c|c|}
) a\\
after\hline
might be enough.Finally: May be do you are searching for a float with subfigures instead of a table with figures? In this case, see the package
subcaption
.