You should organize your Part's resources, such as images, css, javascript, and templates the same way as
CodeBehind Page resources.
Create a single folder named "\bin" in your parts folder with a seperate "images", "files", "css", "javascript", and "templates" folder.
An icon for your part (optional) should be placed in \bin\images\icon.png
A preview screenshot for your part (optional) should be placed in \bin\images\screenshot\small.gif
Your final directory will look something like:
partname/
|--partPartname.php
|--info.php
|--bin/
|-- files/
|-- images/
|--icon.png <-- optional
|--screenshot/
|--small.gif <-- optional
|-- css/
|-- javascript/
|-- templates/