hardeyboyz I think you have misunderstood. First of all, you need to put the custom preview image in the same dir as the file or folder you are creating the preview image for. That is the only way we can link custom preview images to files or folders.
For example, if you have folder name download, you must create an image file _files_download.png (or jpg). Both the folder and the preview must exist in the same dir:
download
_files_download.png
For example in our demo we have:
custom previews/folder1
custom previews/_files_folder1.png
There is extensive documentation in the custom previews readme file:
https://demo.files.gallery/?tests/custom%20previews#readme.md