Rule #1 of promo:

If you’re already rolling your eyes at me for stating the bleeding obvious please click away now. This post is not for you. But I’m amazed how often I see people breaking this golden rule.
Don’t post something that says, “My new book is out today!” with a picture of the cover, or a beautiful graphic, but with nothing to click on.

People are busy and you want to catch their attention. Most people won’t bother to go and search for your book unless they are a die-hard fan, but if you make it easy for them to click through to the sales site they might be tempted to buy it.
Preferably post a direct link to a sales site rather than a link to your blog because it’s less effort for the potential reader (the less clicking they have to do the better). I prefer to link to Amazon because it’s a one-click purchase there. You want to make it as easy for your potential readers as possible to buy your book!
The other reason you want to provide a link in your social media post, is that people are more likely to share it. There’s not much point sharing a post of someone else’s book cover, no matter how beautiful it is. But if there is a link to pass on, then people are more inclined to hit RT/share and spread the word.