Heh, I finally found a way to use my own customized header picture for the journal! This proved a lot harder than I originally thought since I'm using the Chopsticks theme of the Expressive layout. I love the color and overall look-and-feel of this theme. But this particular style turned out to have less straight-forward customizable options. It took me some time to figure out how to achieve what I want by specifying a Custom stylesheet through Custom CSS under the Custom Options tab on the Journal Display page (accessed by selecting the Customize Journal option from the menu bar.) Phew--that's quite a mouthful. I know this is dorky but I want to record what I did so I can remember later on...
While changing the header picture, I also modified some other aspects of the theme. Here's a list of what I'm able to do:
Use my own banner instead of the original chopsticks and teacup picture in the header.
Change the bottom border of the header section to black (it was originally white). I like how the thin line giving a clear definition to the bottom edge of the header, thus separating it from the journal's body.
Remove the thin white border around the header section. This was originally a style set at the layout layer. All themes under Expressive layout have a 1 pixel white border separating adjacent sections. Since the Chopsticks theme is black-and-white-ish, I find the border rather pesky. The theme previously don't have this problem because the original header picture has white background so the border doesn't show.
Change the color, size, and fonts of the text in the header.
Change the color of the navigation bar in the header to match with the text color.
Change the size and font for the journal entry titles.
Get rid of the pesky black border around my user picture since the avatar I use already has a border.
Even with the things I'm able to modifty, I'm still left frustrated with one particular customization I haven't figured out the "how-to" yet: place the navigation bar under the journal title/subtitle instead of above! I've tried different things for weeks now and still coming up blank! ::sigh:: At this point, I'm throwing up my hands and admitting defeat unless someone can enlighten me on what to do.