When you want to create a new post with pictures, there is a tab "Upload a File" click on it and it should lead you to your documents/ pictures files depending on your OS.
If you are responding to a post at the bottom right the "upload a file".
double Click on the picture you want and it will upload.
You can select size such as full or thumbnail. but both selections will allow full size viewing difference is thumbnail opens the photos in a seperate window and full posts them open already.