In frames, illustrations don't just need to be hung up on walls in contemporary white or black frames. Antique frames you can find in markets, charity shops or in your attic - forgotten - can be used. If wood, you can paint them white and distress the edges with sand paper to give them that old edge.

Antique Framing:-

Contemporary Framing:-

One cheap, good way to decorate a Nursery is by using illustrations from classic children's story books.

This classic Tenniel illustration of the "White Rabbit" from Louis Carroll's "Alice's Adventures in Wonderland" could be the ideal decorative illustration for a Nursery wall. 

Framed in a plain white frame with a variety of other Tenniel's classic illustrations could make up a montage of the famous children's story in a way which won't include paint or wallpaper.