a. When using Phonegap Build, your icons and splashes are anywhere you like. PGB doesn't require a specific directory structure of your zip file. Nor does it require some naming scheme.
Best practice is:
- have index.html, config.xml and icon.png in the root directory of your zip
- have everything else nicely organized, as you see fit
- make sure the paths in your config are exactly correct.
b. Make use of the so-called .pgbomit file to tell PGB to include only directories and files that are necessary for each operating system.
See the Getting Started section of the PGB docs: