A nice little image editor utility that I recently started using is Imagewell.. Thoroughly easy to use. Just drag and drop an image onto its window to manipulate the image. You can rename it, change the format (say to jpg), alter the image size and quality, rotate and even add a watermark.
When it comes to saving the image it has a neat concept of locations. You build a list of common locations, and these can be folders on any local disk, remote disk, or a remote directory on an FTP or SFTP server, and it also supports other special cases like Flikr. I find it extremely handy for resizing huge iPhoto images and then automatically uploading to the appropriate folder on my web site in one click.
Easy to use, simple, fast, no fancy features: To me the perfect Mac app.

