Perfectly cool, but like others here I'm curious how you're going to "out do" RainyMood.com. Personally, since they added the "recommended song" feature, likely in response to people so often comparing songs to couple with it (especially The Fragrance of Dark Coffee as I first saw on 4chan), I can't think of much.
I think the changing images are nice, but I think you'll need to do more. Starting with making everything except the image invisible after not moving the mouse for say, 10 seconds. Second, and while larger, better, consider using gifs instead. Up to 1920x1080, looping gifs of rainy scenes. I'll bet you could find some people to help you with that if you wanted. Let one load up, then cycle to it. I think that alone makes you look like a "premium RainyMood".
Maybe if your sound is procedurally generated, maybe adding interactive parameters like "size of rain" (size of field to sample), "amount of drops", "location" (city, rural, forest, lake), and then thunder variables. Let users set up parameters and share them with hashed urls a la "raining.fm/r/fdDiOd4dfji". And if you do use "location", then try to match the images to that.