Upload it with the manage attachment button located near the bottom of the post page. Or upload it to an image hosting website and insert the link using the image button (mountain with the sun, yellowish).
Shift + Cmd + 3 to take the screen shot. There is a little paperclip icon when making a post. Click it and then select "Choose File" under "Upload File from your Computer." Find the file, upload it, and then attach it to your post.