Hello D!
I'd say the easiest way create your "collection" will be to get into a photo editor (PhotoShop Elements, Gimp is free) and create a new blank image. The three images measuring that Flynn has Atahualpa using "out of the box" are 1300 px wide x 160 px high. But, you can vary both the width and height. The Atahualpa Theme Options says:
Header Images
All header images are located in /[theme-name]/images/header. All images in that directory will be rotated. If you don't want rotating header images, leave only one image in that directory.
* If you chose a fixed width layout, the image(s) should be as wide as your layout width.
* If you chose a fluid layout, the images should be as wide as your "max width" setting.
* If you chose no "max-width" setting, your images should be as wide as the widest screen resolution (of your visitors) you want to cater for. 1280 pixels is common today, so the images should be that wide or wider. The next common screen widths are 1440, 1600, 1680 and 1920 pixels.
You'd then open your pictures in the editor. You'll have to exercise some care to insure that the individual images are correctly sized so they'll "fit" into the 1300x160 blank. Then, one by one, copy and paste them into the 1300x160 blank.
From personal experience I can tell you that if you set Atahualpa up with flexible width, as the browser window is narrowed a portion of the
right side of the image will disappear "
under" the right window edge. So anything importgant in the image should be kept nearer the center or left side. You can see how this works at my site http://hatchtown.com/ -- change the width of the browser window to see how the picture gets cropped. (note: I have things set up with a Min and Max width so it's sort of semi-flexible).
The way Flynn has the images/header/ subdirectory working is neat. You could create three different header pictures using maybe 18 or 20 of your pictures and Atahualpa will cycle through the three.
If you're not really conversant with any photo editors maybe the program you do use to "manipulate" or store your pitctures has some sort of "make a collage" feature. You could tell it to make you a collage of the right size to be used as an Atahualpa header.
Let me know how you're getting along. Jim