You dont need special software to take In-game screenshots
Depending on configuration, you can take screenshots by pressing the "-" key on your keyboard.
However, i've noticed sometimes this is different for some people. What you can do is look in your options and look for key-settings. Look for the screenshot one. If its not bound to a key, set it to something you can use.
Once you press this button in-game, it will take a screenshot and add it to your EQ-Directory. Screenshots are pretty large though, almost 1 megabyte each due to being exported in .BMP format, but this makes for better quality pictures which can be changed to .JPG or .GIF to lower the filesize by using image editors.
The filenames should be : EQ000001.BMP or similar. Numbers will continue growing as you take pictures.