If you are on a computer, here is the process:
First you go down to the reply screen and you hit this icon:
Next you click Browse:
Next you click Add This File:
Now you can then go back and get two more pictures to upload a total of three pics. I haven't had any luck doing more than three cause it wouldn't work. But once you have the images you want, you click Save All Images:
Now if you are on a smartphone, I have only had luck by using Imgur and just coping the IMG code and pasting it into the reply.