images in media directory are not writable

When I try running the bulk optimizer on the directory, I get this:

uploads/2012/10/gregory-150×150.jpg is not writable

I get the same error message for every single file. I know this has to be a permissions issue, but I don’t know an easy way to get it to work automatically so that I don’t have to do any manual work each time an image is added. Can anyone help with this? Thank you.

https://www.ads-software.com/extend/plugins/cw-image-optimizer/