Here is another way to do it just within the post you are making. If you have your post in the Advanced format and if you scroll down below the typing area for your post you see the Additional Options you can add pictures from there.
Click on the Manage Attachments button and the following will appear:
Click on the Browse button and search on your computer for the picture file you are looking for.
Once you select it and then click "Upload" it should then upload to the website and it should show up in your post like this: