cpbitmap.github.io is a free browser based tool that can convert .cpbitmap files to multiple image formats.
The .cpbitmap file format is a proprietary format created by Apple to store iOS background images. The most common examples are the LockBackground.cpbitmap and HomeBackground.cpbitmap files found in iPhone or iPad backups.
If the output image is corrupt or looks like it has been stretched and distorted, try selecting another iOS version and exporting again. The.cpbitmap format has changed slightly over the years and may change again at some point in the future. This tool has been tested for iOS versions up to iOS 15. If this tool is no longer working with a new version of iOS, consider opening an issue on GitHub.
If you encounter an error along the lines of RangeError: The value "..." is invalid for option "size", the .cpbitmap file may be from an encrypted backup. Consider using this free tool to extract the .cpbitmap files from the encrypted backup. The .cpbitmapfiles are stored in the Home/Library/SpringBoard path of your device (accessible under "expert mode" in the free tool linked above).
Apple have never released any documentation about the layout of .cpbitmapfiles. We only know how to convert .cpbitmap images due to some clever guessing and trial and error by Simon Whitehead in 2011.
There are a few scripts that use his description to convert .cpbitmapimages to other formats, but there doesn't seem to be a working browser based solution. The code used in this tool was originally written by dr15 in this Stack Overflow answer to run in Node.js, so it has been slightly edited to run in the browser.