You may need to click on the "Go Advanced" button in the lower right corner. You should see a little icon that looks like a paper clip. Clicking on that pops up a window that allows you to upload your image and that will automatically create a thumbnail folks can click on to see the full size you upload. If you want to see what your post will look like before you "submit" it just click on the "preview" button and viola.

I hope that helps.