I don't really get what you're saying but...
Do you mean desktop backgrounds?
To change your desktop background to a Apple default background, step by step:
1.) Go to system preferences
2.) Under personal, it will list several icons. These include Appearance, Dock, Spotlight, and so forth. The one you want is second in the list. It will say 'Desktop & Screen savers'
3.) It should already be on Desktop (by default)
4.) There will be two drop down bars that will say 'Apple' and 'iPhoto libraries'.
5.) Click on one of the above.
6.) Go to the category that interests you. The categories are: Apple images; Nature; Plants; Black and White; Abstract; and a Folder tagged as 'Picture Folder'
7.) Click an image that appeals to you.
8.) Simply click on it. Voila.
If you want a custom image, you will find the image you want somewhere, such as the internet. You will 'right click' this image. You can right click by either pressing the one click mouse, and pressing control at the same time, or by going to system preferences and changing your mouse options there (I'm not going to get into the details of that). A small menu will appear, and prompt you to select an option. There should be one that says 'Make desktop background'. Click this, and, voila.
I don't know what you mean by a 'cross' of leopard and windows, I think you might mean bootcamp program running? I'm sure you could find a picture like this on the net, if you really wanted to make it look like a mac was running windows...