It takes longer and longer for Engines to index sites due to all the submissions. The good news is there are a few things you can do. When a site is submitted to a search engine, the search engine sends out a 'bot', or computer program that surfs the site to see what it has. The bot looks in a number of areas on the page for information on how it should rank the page. The bot is really trying to find out what the page's content is about.
Different bots look at different areas of the page.
Some bots might also look at alt-text. Once the bot has finished 'crawling' or 'spidering' the page, it has the information it needs to index the site appropriately. Once the bot is finished with that page, it will look for links within the site. If it finds one, it will follow it and index that page. The more links the bot finds, the more pages it indexes.