Yes. It is still effective but you should care about the backlinks quality. If you have high-quality backlinks then your website will rank better in search engines.
Guest posting is same as posting or publishing blog posts. But in this case you will send your blog content to the website where blog owner allows other blogger to post their content on his blog. So when you will send the blog content and he will publish it on his blog then it will be your guest...
One more thing which comes into images optimization is image size. We should reduce the size of images so that the page can load faster. It has become more important now.
Keyword research is used to find the best keywords so that your website get good traffic and leads. If you are doing SEO for a website then the main goal is to get visitors on the website and sale the product or services provided by the business.
From my opinion both are equally important for better search engine ranking. however in current scenario there are many factors which can affect your search engine ranking are responsive web design, website loading speed. So we should also keep our eyes on these factors.