Most definitely yes. You can indeed set the background of your Gmail pages to be the photo of your choice, either through grabbing it out of your Picasa album, if you have one, or simply uploading the image directly to Gmail. The trick is to find where to make these changes...

If you're a long time Gmail user like I am, then you'll remember that on the top right it used to say "Settings", which made it really easy to work with. The streamlined redesign, however, tucks a lot of those important options into the menu below the "gear wheel" icon on the very top right corner.

To set this, then, you need to start by clicking on the gear wheel and getting that menu to show up:

Choose "Mail Settings" and you'll get to the surprisingly complex Gmail settings area with the zillion preferences you can configure. Look along the top, however, and you'll see all the options shown in one place:

You want "Themes", as you've already figured out if you've been trying different Gmail themes. Click on it, then scroll down to the bottom:

At this point you'll want to click on "Create your own theme" and a new window pops up:

All things considered, this is an alarmingly boring window, but don't worry, you can tweak and fiddle to your heart's content to get things looking the way you prefer.

To set a background photo, you need to basically play: you need to move your cursor over the bottom portion of the window, at which point a region suddenly is highlighted and there's a small pop-up menu button that appears:

Click on "Select" and you'll be able to choose between Picasa and your own computer:

Since I'm assuming that you don't already have the photo you want in Picasa, press "Choose File" and go through the file select dialog window to pic the photo that you'd like to use. Upload it by clicking on "Upload" and the little preview window will show a snippet of your photograph:

Before you leave this area, I'd suggest you set Tiling "Both" so that the image will spread to cover the entire background regardless of size. Done? Click "Save" and...

Very nice background graphic. Now I can go back and tweak the text link colors, because it's clear that dark blue on the green grass photo is, well, completely unreadable. :-)