With respect to the problem, first consider that the figure environment is a float, that mean that it can stay here, or float freely to the top, to the bottom, or jump to the next page or the last page, according to complex rules. See What are penalties and which ones are defined? and How to influence the position of float environments like figure and table in LaTeX?
Moreover, a float can add some vertical space around it. See Space after float with [h]
But the float width is always the width of the text, so center it is completely useless. As Jubobs point, what you really want is center the contents of the float, but center add vertical space also, so it is better \centering.
Another problem could be the \includegraphic option. Play only with the text and this command in your MWE, to isolate the problem.
Note that you fix a size relative of the original file size (that is, the 70% of who know ...) so if the image can be fitted or not depend of the original file.
To avoid this, usually it is a better idea fix an absolute width or height size (as [height=10cm]) that you know that is smaller than the page, but much better approach is to fix sizes relatives to the text size area. For example, with [height=.5\textheight] so your image will be the half of the text, no matter if you are using an A4 or an A5 paper, or how wider are your margins. You can fix both width or height and maintain the aspect ratio in this way:
\includegraphics[width=\textwidth,height=.95\textheight,keepaspectratio]
{example-image-10x16} % require the mwe package
What now? The image can be fitted in the page, filling the remaining space. But note that is too close to the text. To take care of this, you can include this image in a float (or for another reason, as include a caption), but obviously reducing .95\textheight value as needed, if you want avoid a float jump:
\documentclass{article}
\usepackage{graphicx}
\begin{document}
This is my attempt to import the image into latex.
I do not know how so I decided to try all possible cases.
\begin{figure}[h]
\centering
\includegraphics[width=\textwidth,height=.75\textheight,keepaspectratio]{example-image-10x16}
\end{figure}
\end{document}
Finally, if the problem is not a proper scaling, but a bad image that contain empty areas, you can crop the image with Gimp or another tool, as suggested in the comments, but also maintain the original image and show only some area. For example:
\includegraphics[width=\textwidth,height=.75\textheight,
keepaspectratio,bb=0bp 0bp 200bp 300bp,clip]{example-image-10x16}
Note: For a MWE with example images, use the option demo in graphicx package or install the mwe package to use some standard images as example-image-a, so others can deal with your code without downloading any image (May be you will need include \usepackage{mwe} in the preamble to use the example images, but at least in Linux with TeXLive 2013 is not needed.)
\begin{center} \begin{figure} \includegraphics[scale=0.7]{12.pdf} \end{figure} \end{center}, write\begin{figure} \centering \includegraphics[scale=0.7]{12.pdf} \end{figure}. – jub0bs Feb 13 '14 at 09:16@echo off echo "Cropping pdf files ..." for %%i in (./*.pdf) do pdfcrop %%i %%~ni-cropped.pdf --verbose echo. echo Done pause– Bernard Feb 15 '14 at 00:23-croppedat the end of its name. – Bernard Feb 15 '14 at 00:31