Wasn't he turned because of the birds he found in his garden and since then a patron for animals? I could be wrong but I remember reading something like that in my art books. If this is the guy I'm thinking of why not add suggestions to that story? I think the tree and garden is already there. You could add birds but I'm not sure how much that would crowd the cover. How would it look if the man would be a bit smaller? Not sure these are good tips because I don't know what size the front cover will be.
Could you make indications or lines to make that clearer for us?