The simple answer is no – search engine submission isn’t necessary. The majority of search engines nowadays (most notably Google) crawl and index pages by following links. Using that logic, a single inbound link from any already-indexed page will identify your page to the engine. Subsequently, if that page links to other pages within your site, they will also be indexed… and so on.
For this reason, inbound linking is very important. In fact, acquiring back-links may be the most important of SEO. In theory though, a website owner shouldn’t have to ’scout’ or ‘hunt’ for links. If the presented content is of interest, useful, and/or important, there is a natural tendency among web users to link to information. This is the basis for the Google PageRank algorithm.
With all due respect, submitting your site to the search engines can’t hurt. Plus, it only entails about 5 minutes of added work assuming you only submit to the big guys. And once is enough… you don’t need to submit your site more than once. It doesn’t help or get your site crawled any quicker or more often.
When assessing page structure and layout, there is a subtle, yet strategic way to use images in an SEO-friendly manner (beyond ALT tags) that improves your search rank while allowing you to integrate the necessary marketing message(s). Confused? Let’s look at an example:
Suppose you operate a travel site and you want to optimize a given page for the term “Las Vegas hotel”. Suppose that you also want to include an enticing marketing message such as “Book now and save 20%!”. The aforementioned tagline lacks descriptive text, but possesses persuasive characteristics. That being said, you may want to place the tagline in an image and the key phrase (i.e. Las Vegas hotel) in a header tag. This places emphasis on the desired term, yet still provides a marketing opportunity without compromising keyword consistency.