JP, As Ken says, clicking on the thumbnail of your picture opens it up in the browser sufficiently large. If you click on the file name it asks you to download it.
However, there is also another way to embed pictures in posts. You chose to attach your picture using the "Attach File" button. You can also directly embed a picture using the "Embed Local Media" tab, which is the penultimate icon on the tool bar. It looks like a piece of film. If you click on that you can embed a picture directly into the textbox. You will have the option of about 14 different picture sizes. The medium option looks like this: