Every time I find a great looking cover for a book I shouldn't have to do some 5d chess shit to figure out who drew it.
Writers: Feature their name, prominently, in every single space the cover is shown, even if it's cropped. Even on social media. If you can't fit it in the post then crack open some free editing software and put their name on it near the top or bottom. Their image is selling your book, the literal least you can do is make sure people know who drew it.
Fellow illustrators: make sure whoever you're working with credits you. Sometimes it'll be called a 'credit line'. You can ask for them to add your website alongside your name too even! Just make sure your name is on things- no one is gonna go digging using a tiny, illegible signature in a corner as a basis.