uploaded images default to 600 permissions, but need 644

Hi,
I am new at wordpress, so any help would be appreciated! When I upload an image to a blog post a thumbnail is created and a full resolution image. The thumbnail does not display in the admin section, or on the post page, so I looked at the permissions.
I found that by manually changing the permission to 644, it worked fine, but how can i avoid doing this every time?
Thank you!
Daniel