One of the side effects of higher-resolution displays on mobile devices, such as the new iPad, is that compression on web images is more pronounced than ever. It's been discussed at length and even joked about in webcomics. There's no denying that compression in jpegs and gifs really stand out on high PPI screens. But web designers can't just save all their images as uncompressed PNGs--bandwidth is as much as premium on smartphones and tablets as it was on desktops back in the days of dial-up and ISDN. And as web developer Jeremy Keith points out, the art of image optimization is back in fashion again. Finding that perfect balance of image size and compression quality improves the overall user experience to a website or mobile app, and it's more than just hitting that "save for web" option in Photoshop. Here are two freeware tools that I recommend for web designers (or anyone that edits images for web publishing) that can squeeze extra bytes out of JPEGs, GIFs, and PNGs without extra lossy compression. They work by running image files through multiple open source algorithms that remove unnecessary metadata, gamma and transparency channel data.
For OS X, ImageOptim is an extremely elegant lossless compression tool that is as simple as drag-and-drop. Just dump your files in the window and let it work. The freeware app runs each file through four different compression algorithms, though you can opt out of the slower (typically better) ones to cut down on compression time. Your compression percentage is displayed and the original files are automatically overwritten, retaining their original permissions and hardlinks. You can choose to save backups of the original files as well. ImageOptim's creator explains more about the nuances of PNG optimization in this post.
On Windows, RIOT (Radical Image Optimization Tool) is my compression program of choice. It also offers drag-and-drop ease-of-use, but gives additional resizing and batch-processing options for bigger projects.
Both ImageOptim and RIOT tap into PNGOUT, the powerful PNG optimizer developed by Ken Silverman (yep, of Duke Nukem 3D's Build Engine fame). PNGOUT isn't included in either of the apps so you have to download and install it manually. It's also very slow to run, even on a quad-core Sandy Bridge processor, so I recommend leaving it off unless you're under severe file size constraints.