Description:
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According To Documentation
" The getimagesize() function will determine the size of any given image file and return the dimensions along with the file type and a height/width text string to be used inside a normal HTML IMG tag and the correspondant HTTP content type. "

Many developers just relay on this function while uploading images. As it is stated that it will also provide content type. And just check extension at client side. But it is very easy to bypass this function and execute commands.

Test script:
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For the test purpose i use gifsicle utility for adding comments into a GIF.
Command
"gifsicle -c '<?php ?>'" anygif.gif > phpgif.gif"

Then while uploading change extension to .php with burpsuite or any other and you had successfully uploaded your shell. getimagesize won't check file extension that's why many websites are vulnerable due to some noob developers and failure of this function. 
I am reporting this to you after compromising security of many websites and i had help them to secure their system.