Image to PDF or XPS is a portable tool which imports multiple image files, and saves them as one or more PDF or XPS files.
The program supports BMP, GIF, PNG, TIF, WMF, EMF, XPS, JPG, JP2 and J2K files. These may be added via an "Add Files" button, or by dragging and dropping them onto the program window.
If the images are very different sizes then this can make for a messy PDF, but there are plenty of options to help. You're able to define your page size and margins, position images in the centre of the page or the top-left corner, shrink over-sized images or enlarge small ones to best fit the available space.
Output options include your preferred format (PDF or XPS), the output folder, and whether you'd like to export multiple files (each image is converted to a separate PDF) or just one (each image is a separate page in a single PDF).
All these settings are displayed on a single dialog box, so you'll need to spend a moment figuring out how everything works. But it's really not difficult, and you'll be creating PDFs within a few minutes.
There are lots of similar tools around, but Image to PDF or XPS stands out for its file format support and page size controls, all packaged in a tiny (91KB download) portable application.