Kraken.io - Image Optimization Web Service
Kraken is a web service designed to perform a similar function to desktop based applications such as ImageOptim. For as little as $7 a month (for half-a-gigabyte of images processed a month), you can have Kraken.io process your images and compress them. Alternatively, you can use their free web service by uploading files individually. The service works significantly faster than ImageOptim because of the powerful servers that they use to crunch your images.
But, how does it compare to the desktop equivalent?
The quick answer is, "it depends." Minimally, Kraken's optimization is nearly as good as ImageOptim, or equal to it. For example, Kraken configured with "Lossless" settings saved an average of 34% off of the original file size off of several PNG images, whereas ImageOptim saved 35.1%. The difference is relatively minor.
In the JPG arena, Kraken initially appears to perform significantly better than ImageOptim, saving 47% of the original file size versus ImageOptim's 34%. However, Kraken obtains these savings by reducing the image quality setting of the JPG. Conversely, ImageOptim has a maximum quality setting, but will not reduce the quality beyond this user set number. Instead, ImageOptim performs optimizations with the file format itself to achieve a lossless compression. This is similar in behavior to Kraken's ability to losslessly compress JPG files.
For example, the following two images were compressed by Kraken's web service-- one losslessly, one with lossy compression. As you can see, Kraken actually does a very good job with its compression in this case and does not introduce any significant artifacts into the image, despite the 73% savings over the original image.
To the casual observer, there is virtually no difference between the two images above. The most noticeable area where artifacts are introduced is around the lights hanging across the road, though it is a relatively minor side effect to shaving off nearly 3/4 of the original file size. You can see this yourself by clicking on the images above and opening the full sized images in new browser tabs, zooming in, and then switching back and forth between tabs.
However, this isn't necessarily true for all images. For example, the following Nike Fuelband+ SE press images have been run through Kraken-- both with lossless and lossy compression.
The lossy compressed image has a significant number of artifacts around the face of the band, resulting in an image that does not look optimal. Interestingly, the lossless compressed image from Kraken is the same size as the image produced by ImageOptim-- likely because Kraken's using the same optimization backend as ImageOptim, with some additional fine tuning.
A point to note is that Kraken does not accept images above 1mb in size for the free web interface, or above 8mb for any of their paid plans. This may be an issue when you're attempting to compress photographs straight off of camera, or large PNG infographics and such. You will have to shrink the files by hand locally prior to using the Kraken web service. In contrast, desktop based apps such as ImageOptim have no limitation on the file size in practice.
Another primary issue with Kraken is the lack of ability to change quality settings. This is especially a problem with lossy compression, where the quality of the image is degraded. It'd be ideal to have a slider to determine what quality to file size ratio is desired.
Kraken's paid plans have several additional features over the free web interface, such as the ability to optimize images by pasting a URL, as well as scan an entire website and optimize all images. Additionally, Pro users have the ability to save images in their Kraken Cloud Storage account.
In conclusion, Kraken is a great web service to replace existing desktop apps such as ImageOptim. Pro users also have the neat ability to use the API, so that images can be optimized on a website's deployment. As long as you're happy with the default settings Kraken provides, however, it's a great service and will help you maintain high performance on your own website.