Combine multiple images into one PDF using Preview
In Preview, choose "File->Open..." and navigate to the folder containing all your images. Once all images are open, click in the thumbnail tray (at the right), choose "Edit->Select All", then "File->Print Selected Items". In the print dialog, choose the PDF button at the bottom left of the window, then select the "Save as PDF..." item. You'll be asked where you want to save the file, and for a file name, click "Save" and you're done.
Be sure you select "File->Print Selected Items" instead of "File->Print" otherwise it won't work.
Good Luck!
Expanded explaination from Google search result at:
http://hints.macworld.com/article.php?story=20090119162659107