(I'll label these as "What's New in CS4". This way, you can just refer to the Labels section (located on the left sidebar) and be taken directly to all posts that reference this subject.)
When you open a photo with a new document already open, this is how your screen will appear.
2a. This simply splits your 2 documents into 2 equally sized windows and allows you to see both of them side by side. Then you would just click, hold, and drag your photo over onto your new document.
3a. To go back to a full screen of your new document, you can either click on the Arrange Documents icon again and choose Consolidate All (which gives you full screen with your workspace showing and the documents arranged in tabs). This will keep your photo open, but out of your way. Or, if you know you're done with your photo, you can click the "x" on the photo tab to close it out.
NOTE: Instead of clicking on the Arrange Documents icon again, you can also right-click on one of the tabs and choose Consolidate All to Here (or your preferred command.)
2b. This pulls it out if the Consolidated Documents and places it in a "floating" position over your new document. Your new document remains "docked" behind this floating window, which means it will remain there. A "docked" document can't be pulled in front of a floating window without floating both documents.
3b. Now you would simply click, hold, and drag your photo over to your new document.
4b. To keep you photo open in case you need it later, you can simply minimize it. Or if you're done with your photo, you can just click the "x" to close it out.
There are several other ways to view and arrange your documents. Now that you know how to get to them, you can play around with each view and see which one you like best. :)
I'll be back with more tips as I learn them. :)