You have a website about cars, and on your website you put an image of a Ford Escort and you make that image clickable, and when people click on the image you want those people to be taken to Ford's website. Of course, you can make people be taken anywhere you want (to a specific page on your website, to your collection of images, to another website, even to google's website if you want), not to Ford's website. I was just giving an example.
It doesn't matter where people go when they click on that image, that clickable image is a link back to something else (a page on your website, or a different website). Therefore it is a BACKlink because it links back to the destination that you specified.
Of course, you can link back to other pages/websites using anything, not just images. You can make text clickable, you can make your own website's logo clickable (actually, many websites do this, they make the logo on their website link back to the front page of their website), you can make an audio file link back to another page / website, etc. You get the idea. Any element (image, text, logo, audio file, video file, table, etc) on your website that can be clicked on and which takes people to the destination you specify is a backlink from one destination (the page where the clickable element is on) to another destination (the page or website that the clickable element takes people to).
Pretty easy to understand