Open Picasa and at the top left choose Import.
This will permanently add this photo to Picasa's library (unless of course you delete it from your hard drive!). If you want to temporarily edit a photo in Picasa and export it but not keep the original in the library you can also choose File>Add a File.
Import the file(s) you want to use into the library. When the import is done, select the photo(s) you want to use in your banner. Essentially what we are going to do is create a "collage". You can do this with 1 photo or many photos.
If you are using only one photo, proceed as follows:
Select the photo so it appears in the picture tray at the bottom left. Go up to the menu bar and click on Create and choose "Picture Collage". A new tab will open within Picasa that looks like this:
(click to enlarge)
On the left, make sure "Picture Pile" is selected as your style. This allows you to move the photo around freely and also resize/rotate it within the collage space.
Click on the page format menu drill down. Choose "Add Custom Aspect Ratio":
Choose that as your Page Format. Next hover over your photo and position/resize it how you want it.
Click on "Create Collage" once you are satisfied. It will only take a few seconds and then it will give you the collage in an edit window:
Once it is saved it is ready to use! You can edit this collage the same as you would edit any other photo in Picasa. Frames, effects, retouching, etc. all of that is available on Collages. There are also more ways than one to do things but this is what I have found to be the easiest!