For that you need an image host. Something like Flickr or PhotoBucket or ImageShack. There's a ton of free ones, just google free image hosting and sign up for an account somewhere you like. Then you upload your pics to your image hosting account. Usually there is a web interface for ease of use.
Once your picture is uploaded, copy the picture's URL. Start (or edit) your post and click the INSERT MEDIA button (11th from the left). Then paste the picture's URL into the box that pops up and save it. It'll place your picture in the message window and then you can resize it manually to fit by dragging the corner and edge boxes. You'll see what I mean.
Good Luck