ImageKlebor from Shoran Software is a free tool that combines multiple image files into a single image. It has a command-line option that makes it useful for automation and pipeline integration, and it's free to use, even commercially. However, most folks will find that ImageKlebor is best suited for assembling multiple images into one picture, such as scans of LP album covers; they're too big for all but the largest scanners, and most folks end up scanning covers in two passes and then trying to put the two together in an image editor. With ImageKlebor, putting chunks of images together into one picture is both simple and precise.
ImageKlebor has a unique and interesting interface built around a gridded main window and an image tray. The image tray holds image files you can browse to or simply drag and drop into the interface. Sliders let you zoom both the grid and the image tray, and there are numerous other options for configuring the grid; indeed, it's the heart of ImageKlebor and where you start with its controls. Since we hadn't created any projects yet, we opted to start with the software's default grid. We opened a folder of scanned drawings, dragged four into the image tray, and then added them to the grid. Right-clicking a blank grid calls up a menu of options, including the ability to leave grid cells blank. We clicked Combine Image, and ImageKlebor quickly assembled the grid into a single, seamless image in the output directory. You can specify options such as Region, which only uses a specified part of the image, automatically calculated; and Overlap, which lets you specify how many pixels of each image overlap each other. You can also stack images. The program's PDF-based Help file offers clear explanations of everything, but ImageKlebor is pretty easy to use, once you get the hang of how it works.
This interesting tool takes a fiddly, complex process that almost never turns out quite right and makes it as simple as drag-drop-click.