You can do this with an image block and the "image links to another page option" but first you have to go to your sitemap, and than select the options button and than select show systems pages

(screenshots attached)

Now that you can see all systems pages you can see the login page in your sitemap and when selecting the link to another page option from the image block (or most sliders) the login page and other systems pages will be visible.

Alternatively you can add the image through a content block and then click the image, hit the link button and enter yoursite.com/login or yoursite.com/index.php/login if you havent turned on pretty urls.