If you open up your chat and you want to type something, you will see this little thingie.
Press on it, paste the URL (MAKE SURE IT'S A .PNG FILE!) into it, and there you have it.
For example, you see a cute picture, press "Show Image" , you will see the URL of the picture (Note, this is .jpg, but it also works. / .png & .jpg are the same kind of.)
Press the little Icon, paste it in there and press OK.
Have fun!