E-mage is described as 'From developers to developers: a cross-platform tool for losslessly image compression'. There are more than 10 alternatives to E-mage for a variety of platforms, including Windows, Mac, Linux, Online / Web-based and Android. The best alternative is Caesium Image Compressor, which is both free and Open Source. Other great apps like E-mage are RIOT - Radical Image Optimization Tool (Free), ImageOptim (Free Personal, Open Source), FileOptimizer (Free, Open Source) and TinyPNG (Freemium).
Caesium Image Compressor lets you compress your pictures up to 90% without visible quality loss. Providing a simple yet effective interface with a real-time preview and multiple image processing at the same time, everyone will be able to reach the best result.
Saving images for web/email/other media with the best compression ratio and quality is a difficult task if you don’t have the right tools. Choosing the right format then choosing the right parameters are the key factors of smaller files.
ImageOptim is excellent for publishing images on the web (easily shrinks images “Saved for Web” in Photoshop). It's useful for making Mac and iPhone/iPad applications smaller (if you configure Xcode to allow better optimization).
OptiPNG is a command-line PNG optimizer that recompresses image files to a smaller size, without losing any information. This program also converts external formats (BMP, GIF, PNM and TIFF) to optimized PNG, and performs PNG integrity checks and corrections.
PngOptimizer compresses graphics files and removes extra, unnecessary data without affecting quality. It will update PNG files or converting BMP, GIF or TGA files to the PNG format using simple drag-and-drop.