How would it effect SEO, if I were to delete unused images from the website? (WordPress)
I'm going to assume you have a web page with several images and you used wordpress to remove them off the page, but I'll answer from the basic webpage perspective (never mind wordpress, joomla or other CMS's).
If your page had an image defined by a friendly URL with quality description with it (as defined in its alt tag) with quality paragraphs and you removed the image then your page may have a harder time appearing in search especially if the image was what the guest was searching for.
If on the other hand, the image was just a random decoration on the site and the image URL was something not so meaningful (example: http://example.com/images/corner.jpg) or the image was not well described (example: the alt tag was missing), then chances are seo will not go against you.
If the images are indexed on Google and if you are driving traffic from them, I don't think to delete them is a good idea. The best thing you can do is to check your Google analytics first and delete only those images which do not bring any significant traffic or are not indexed by Google.
In short, no.
Generally if you're got more post content and your site is not about image sharing, then generally you should be okay.
In fact deleting unused images would only increase page load speeds and thus increase SEO.