The best file format for storing such large images largely depends on your needs. This includes the balance you want to strike between file size, image quality, compatibility, and whether you want to preserve all the metadata that comes with the image.

Here are a few options:

- __TIFF__ (Tagged Image File Format): TIFF is a flexible and adaptable file format that can handle images and data within a single file, including multiple images, and even data in different formats. It is lossless, meaning it does not lose quality when the image is saved, and is therefore suitable for storing images at high quality. TIFF files are larger than most other file formats, which could be a disadvantage considering the already large size of your 400-megapixel image.
- __PNG__ (Portable Network Graphics): PNG is a lossless compression file format, which makes it a common choice for use on the web. PNG is a great format to use if you do not want to worry about the loss of quality associated with JPEG files. However, PNG files can be larger than JPEGs and may not be as well suited to extremely high-resolution images like your 400-megapixel image.
- __RAW__: If the image was taken with a digital camera that supports this format, storing the image in RAW format would preserve the most possible information. However, RAW formats are usually proprietary to specific camera manufacturers and are not standardized. They are also very large files and not suitable for sharing or web use.
- __JPG__ (Joint Photographic Experts Group): JPG is one of the most popular and widely used image formats on the web and other platforms. It uses a lossy compression method that reduces the file size by discarding some information and quality from the image. However, JPG has some limitations and drawbacks that make it unsuitable for storing very large or high-quality images. For example, JPG has a maximum file size of 4GB and a maximum resolution of 65,535 x 65,535 pixels. JPG can also introduce artifacts or noise in the image due to the lossy compression process.
- __JPEG 2000__: JPEG 2000 offers a significant improvement over the original JPEG standard and supports a much larger maximum file size. It uses a more sophisticated compression method that can produce better-quality images at smaller file sizes. JPEG 2000 also supports lossless compression. However, it is not as widely supported as ordinary JPEG or TIFF files.
- __HEIF__ (High Efficiency Image Format): HEIF is a relatively new format that offers better compression than JPEG and more color depth. It also allows for the storage of multiple images in one file (which is great for things like Live Photos or bursts), as well as other types of data. However, as of my knowledge cutoff in 2021, HEIF is still not universally supported, particularly on older devices or software.
- __PSB__ (Photoshop Large Document Format): If you are using Adobe Photoshop, you might consider PSB, a format designed for extremely large images. PSB supports images up to 200,000 pixels in any dimension, up to a total of 4 Exabytes (though the saved file size is limited to 2GB if you want it to be compatible with older software).

Choose the format that best fits your needs in terms of quality, file size, and compatibility. For pure image quality and data preservation, TIFF or RAW is a good choice. For better compression, consider JPEG 2000 or HEIF (if supported). If you're working in Photoshop, PSB might be the best choice.
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