As it is obvious by title, I need to add a diagram to clarify my intuitions in some questions. How can I do it? Is there any limitation in format and size of the pictures?
From the formatting help page:
Images can be added primarily by using the the editor toolbar button . This brings up a special interface that allows you to upload an image online (through the imgur hosting service) through us - even from your clipboard. Alternatively, it can be input similarly to adding a link: