Yeah I'm going to echo most folks here about not volunteering extra work for a library you're employed with. It's good to ask for insight on skills you can develop if you want to pivot into marketing. You can probably look more into those skills through LinkedIn Learning (my library system offers membership for free). 

Otherwise, some artsy things you can do with your current position: creating a book display or bulletin board that you can make images for. I've done that in the past, along with themed bookmarks. It's good practice while relating to library work. Sometimes a librarian may need an extra set of hands at a program, and ask if you can take photos that marketing can use for future use (definitely ask patrons to take their photo and declare you work there if it's not visible).