Squash (was Squish) is a super-simple app for compressing your images for lower filesize! Don't lose quality, or end up with grainy or horrible images. Squish maintains their existing resolution and quality, and just reduces the filesize! Whatever your reason, Squish is great for optimizing images for:

Put your photos on a diet with JPEGmini! JPEGmini reduces the file size of your photos by up to 5X, while keeping their original resolution, quality and JPEG format. With JPEGmini you can optimize all your photos to free up valuable hard drive space, and share full-resolution photos with your friends much faster.