It's best to start working on your content (copy/text and images) as soon as you can. Getting content from our clients is the top reason for a delay with a project. We understand that not everyone is good at writing and that's why content takes a while. Incidentally, we do offer copy writing services if that will help to get this task done and if you want effective copy on your website.
Within your project on Basecamp, there should be a working site map document. We will need content for each item on that list that is a true page (versus a placeholder). The site map is a guide as to what content the website will have, so start there.
Do it in a format that is clear. What we prefer are separate documents - one for each item in the site map. For example, if there's a page called "About Us" then send us a document named "About Us.doc" that contains the text (and images) for that page. If you want to send us images separately, that's perfectly fine. Just reference the image names at the end of the document so that we know which images you want on that page.
We would prefer getting all the content at once but if you have a good chunk of it done, then go ahead and send it to us so that we can start working on it. All content submitted is final, so if we have to re-do any page, then there could be extra charges.
To upload your content, simply upload it in Basecamp so that everyone on the team gets the content.