input: Write a python function to prepare a presentation in PDF format using the reportlab library.
  assert prepare_presentation(['slide1.png', 'slide2.png']) == 'presentation.pdf'
  assert prepare_presentation(['intro.png', 'content.png', 'conclusion.png']) == 'presentation.pdf'
output: def prepare_presentation(images):
      from reportlab.lib.pagesizes import letter
      from reportlab.pdfgen import canvas
  
      pdf_file = "presentation.pdf"
      c = canvas.Canvas(pdf_file, pagesize=letter)
      width, height = letter
  
      for image in images:
          c.drawImage(image, 0, 0, width, height)
          c.showPage()
  
      c.save()
      return pdf_file