Creating and testing desktop pics

[mediafred]

I want to make desktop pictures for different Droid phones.

1. How do you make one that sizes nicely for all the different models? Do you have to make a separate one for all of them? Or can you make one that fits the Droid X and Evo and it will just scale down nicely for the Incredible, Droid, etc.? Or is this a bad idea?

2. Is there a way to test on a computer how the different images will appear in the different phone screens without having to buy them?

Thanks.