When you upload your picture you would have a URL of your picture. All you need to do know is <img src="YOUR PATH">. You have to add this to the edit HTML window of the GPC. As you click the Save changes the picture should take a some time to load and it will appear.
You can also use pictures as links to sites. When you add a picture two pieces of code appear. The first part with the <a href should be deleted since that part links the picture to the actual file. After you deleted the first <a href and the </a> part you will be left with the img tag and that part should be something like this:
<img src="yourImage.xyz>
Then you have to re-add the <a href tag. In the href="PATH" part you will need to replace the PATH with the URL of the page that you want your image to link to. If you want it to link to one of your pages then, all you need to write in the href property is the URL of the page, without the "http://USERNAME.googlepages.com/" part. Inside the <a href tag you paste the picture code. Of course you need to do all of this inside the Edit HTML window of GPC. Then save changes and the picture should be a link. Example given below:
<a href="http://oregac.googlepages.com/linkback"><img src="http://oregac.googlepages.com/linkback.png"></a>
Taking the above code, you would replace the blue part with your link, and the location of the image.