minimal carts - maximum customization?

[tino.schlegel]

I don't need ANYTHING on the page from the default template except:
a center column for the products
a right sidebox shopping cart, (no left sidebox/column at all) and -
the UPS module for shipping calculation.

Great! Dont worry about things that you dont need. Just deactivate them.

You can easily disable the right column in the Admin Configuration > ... > Disable Left Sidebox Globally or just dont activate any sidebox on the left column.

I want my own background image - a simple gradient - tiled down the page, and the only items on the page would be:
1. a header image,
2. a footer image, and
3. the product line-up.

Your customizations fall under your so called "just header and color scheme variations" category.

1 + 2 can be changed in the stylesheet.

3. is in the Admin section of your shopping cart.

I don't want colored backgrounds for the columns - I want the background image to show through.

Excellent! Don't assign a color for the columns in your stylesheet.

As I plow through the customization and get rid of all the stuff I don't want, the more I pare it down the more it seems like I won't be able to achieve this goal.

I wonder why. How far have you gotten with your customization? Maybe post a link of your website.

All the Zen Carts I've seen have looked more or less the same, with just header and color scheme variations. None seem to have gone as far as I want to with customization. Some things just seem like they are not editable.

Now, I dont know which carts you have seen, but many websites have customized their Zencart a lot and they look very nice.

So now I'm beginning to wonder if Zen Cart is the cart to use.

You are strongly encouraged to try other shopping carts to learn your own lesson about individual customization.

Apart from the left sidebox no showing, the rest is more or less CSS customisation, smart use of images and moving sections of php code in template files to suit your individual website arrangements.