var updateCount = 0;
var updateAmount = 10;

function showImage() {
  var ss = SpreadsheetApp.getActiveSpreadsheet()
  var formulaSheet = ss.getSheetByName("Sheet1");
  var countCell = formulaSheet.getRange("B2");
  var formulaCell = formulaSheet.getRange("B1");

  if (updateCount < updateAmount) {
    updateCount++;
    countCell.setValue(updateCount);

    // Delay 1 second
    Utilities.sleep(1000);

    if (updateCount % 2 )
      formulaCell.setFormula('=IMAGE("https://ncviewer.com/img/GithubBanner.png", 1)');
    else
      formulaCell.setFormula('=IMAGE("https://ncviewer.com/img/ncviewer.png", 1)');

    // Loop
    showImage();
  }
}