Thanks!!!
If you already have a file in your pc, you should upload them first in a site like postimg.org and then, copy the link and use the function "INSERT" of this post Box, Choose the "insert image" in it´s menu and past the web link on it.
If you found a pic in web, but you do not want to do all the process of save and upload, click over the pic with the mouse´s right button to seek the options, scroll down to find "Properties" of the picture and it will be displayed the info of the pic file like size, type of file and of course the web address used on it. Copy and paste it in the insert image part as I mention above.
I hope this helps