From internal build 7.3.5613 configuration for the hash algorithms is supported. Switching between them is done through the backend configurations(Settings -> Advanced -> Libraries -> Images -> Image URL Signature hash algorithm) or by editing the LibrariesConfig.config ( App_Data/Sitefinity/Configuration).
If Images are generated with one of the hash format and then the algorithm is switched the thumbnails will be broken. In this case, recreate your thumbnails or set enableMultipleHashAlgorithmsSupport =”true” that will check if the hash is valid for both algorithms SHA1 and MD5.
There is option to use both the algorithms in case of thumbnails generated between 7.2 Hotfix 2 and 7.3 release.
The option is not visible in the UI and can be applied only through LibrariesConfig.config ( App_Data/Sitefinity/Configuration) by setting enableMultipleHashAlgorithmsSupport =”true”
In this case the thumbnails do not need to be recreated.