The code below generates the IMAGE function for the sheet to show thumbnails of all (PDF) files in a chosen folder, obtained with a URL and the file ID:

```
function scannedMail() {
  var files, file, sheet;
  sheet = SpreadsheetApp.getActive().getSheetByName('ScannedMail');

  files = DriveApp.getFoldersByName("ScannedMail").next().searchFiles('');
  var i = 1;
  while (files.hasNext()) {
    var file = files.next();
    var ID = file.getId();
    sheet.getRange('A' + i).setValue("=IMAGE(\"https://drive.google.com/thumbnail?authuser=0&sz=w320&id=" + ID + "\"\)");
    sheet.getRange('B' + i).setValue(file.getName());
    i=i+1;
  }
}

```

Yet it does not show the thumbnails. I found out that it shows just the ones where I manually retrieved the ID from getting a "shareable link". Apparently this ensures the right share settings to get the thumbnails of my own files.

1) Is the previous assumption correct, and why do I need to adapt share settings somehow, where I have read other files without any issues?

2) How can I adapt the script to adapt the share settings, or make it work otherwise?
The script is meant to operate just within my own Google account, and to keep the files private.

I tried sharing the folder with myself, but that does not make a difference (or sense). Is the script somehow regarded as being another user than myself?

Following suggestions from @Rubén and @Cooper, I have tried using `insertImage` either based on a URL:
 `sheet.insertImage(file.thumbnailLink, 1, i)`
or based on a blob:
 `sheet.insertImage(file.getThumbnail(), 1, i)`

But the most I could get out of Google was "We're sorry, a server error occurred. Please wait a bit and try again", with the code below:

```
function ScannedMail() {
  var files, file, , name, blob, sheet;
  sheet = SpreadsheetApp.getActive().getSheetByName('ScannedMail');
  files = DriveApp.getFoldersByName("ScannedMail").next().searchFiles('');
  var i = 1;
  while (files.hasNext()) {
    file = files.next();
    name = file.getName();  //not needed, just for debugging
    blob = file.getThumbnail();
    sheet.insertImage(blob, 1, i);  // it runs up to here...
    i = i + 1;
  }
}

```

The code execution gets stuck on the first occurrence of `insertImage()`.
So we have 3 approaches (`IMAGE` function in sheet, `insertImage(URL,1,1)`, and `insertImage(blob,1,1)`) but all 3 do not make a thumbnail appear, apart from the first method when you make the file public (not a serious option).

I don't see a duplicate question and answer that helps me find out what is wrong with my code, or helps me to somehow get the required thumbnails in the spreadsheet. The kindly proposed solutions did not succeed in that yet.