Pdf - Printing Images Using A Lot Of Memory?

I'm working with PDFs in VB.NET using a DLL I found on code project:

[URL]

My app allows you to select multiple files in a grid and print them. The files are stored in password-protected zip files, so the first step I do is extract each selected file to a memory stream that I pass to a new PDF wrapper object. Each object gets added to a queue. Then, each object in the queue is printed, page by page, as a system.drawing.image. The whole thing runs on a background worker.

Now, extracting the PDFs to the queue uses hardly any memory. But in the PrintPage event handler, when I extract the images and send them to the printer, something must be going wrong. My memory usage explodes. Each image, of course, is large because it's rendered at 300 dpi, but the memory used by each page isn't being returned to the OS and neither is it being garbage collected. In the end, if I select enough files, I run out of memory. Why?