1. Sitemaps are not required, only suggested*

2. You can update it manually, or look for a 3rd party script or plugin. There are programs that will crawl your site and create a sitemap.

*I use a sitemap.xml for pages but not for images, yet my 7k images are all indexed AFAIK.

I don't use the sitemap.xml for images because they don't change; they don't update. My pages do update so I want to let Googlebot know.

I don't offer a sitemap because my users never look at it.

I've noticed that the crawl on my site takes on a predictable pattern when I don't provide a sitemap. It will be a low amount of data checked daily with a large amount checked every 5-7 days. I'm seeing a lot of HEAD response hits daily on older content and every 5-7 days or so a full page check. The graph looks like quite the sawblade. Newer content is checked more frequently than old stuff.

If you link to a page on your site from another page Google will find it and will decide on it's own how often to check in. A benefit I do see however is when your sitemap shows one version of a url but google finds multiple versions of the page(ie: www, non-www,hashtag and other querie strings) it tends to index the version in the sitemap. Make sure the url in the sitemap matches the canonical if you use both.