Why does picasa always create 2 of every photo?

I am very confused about how iphoto and picasa work together. Because I post many photos to a blog, I must use picasa. However, I do all editing in iphoto. Picasa creates at least 2 of every photo! Can I delete one of them without loosing the other? It bothers me that picasa controls what I can and cannot delete! I would appreciate any help, particularly from any bloggers who may experience this issue. Thanks!