ImageKlebor is a tool that can combine multiple input images (like chunks of a sliced image or frames of an animation) into a single output image. All images can be arbitrarily arranged on a grid (which may also be a horizontal or vertical stripe), and several extended options let you put together your various image slices with ease.
ImageKlebor, which is available for Windows, Linux and MacOSX, comes in two flavors: the graphical (GUI) version lets you put together your images in a visual (manual) way, while the command-line version is especially useful for pipeline integration and automated tasks.
ImageKlebor can be easily used in conjunction with RenderPal V2 to combine sliced images back into one single image; the included manual contains details about how to do this.
And best of all, ImageKlebor is completely free, even for commercial use.