Spreadsheet::WriteExcel actually expects a filename to be passed to ->insert_image(), not raw image data. The following, fairly minimal example works for me:

#!/usr/bin/perl

use strict;
use warnings;
use Spreadsheet::WriteExcel;

my $workbook  = Spreadsheet::WriteExcel->new("Employees.xls");
my $worksheet = $workbook->add_worksheet("Summary");

$worksheet->insert_image('A1', 'Lenna.png');

$workbook->close();
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Since you're pulling the image from a database, you may have to write it to a temporary file. That, or bug the module's author to add a function that takes image data and inserts it directly into the spreadsheet.