img2pdf is a simple commandline utility to convert a list of PNG, TIFF, or JPEG images to a single PDF document, one image per page. It was designed specifically to convert scanned pages to PDF format, but it will accept images of any aspect ratio and scale them to full page size (which, in general, will not maintain the aspect ratio). The hard work is performed by the Panda PDF library and its dependencies.
Tumble converts one or more TIFF (B&W only) and JPEG files into a PDF file. It was developed primarily for use with scanned images. B&W images will be encoded with Group 4 fax (T.6) lossless compression. JPEG images are embedded as-is. Tumble supports multi-page TIFF files, and can generate PDF outline entries (bookmarks) based on the file name and/or the page number.
GraphicsMagick is a robust collection of tools and libraries which support reading, writing, and manipulating an image in over 90 major formats including popular formats like DPX, DICOM, BMP, GIF, JPEG, JPEG-2000, PDF, PNG, PNM, SVG, and TIFF. A high-quality 2D renderer is included, which provides a subset of SVG capabilities. C, C++, Perl, Tcl, and Ruby are supported. Originally based on ImageMagick, GraphicsMagick focuses on performance, minimizing bugs, and providing stable APIs and ABIs. It runs on all modern variants of Unix, Windows, and Mac OS X.