there are 2 ways
1) Press Post Reply and it will take u to the section where u can upload an image from your harddrive and u can attach it provided it under 100 KB.
2) If its another site that has the image, u can use the url of that image and post it within IMG tags
example [*IMG] URL [*/IMG] (without the *)
as for option 1)... here is what u will see when u press post reply.