This is a little command line java program, that takes all images in a folder and sticks them together to one image strip. Online, I could not find a single simple solution for that, so I just created the needed tool.
Simply execute it in the folder with your image sprites (file types can be: jpg, jpeg, png, gif, bmp, file names should be numbers in the desired order) and a file "strip.png" will be created, preserving possible transparencies:
java -jar ImageStripCreator.jar
HINT: If you need a vertical strip rather than a horizontal one, execute it with the parameter -vertical.
java -jar ImageStripCreator.jar -vertical