how can I combine a lot of pictures into one perfectly?

[kenny1999]

for example
four images to be combined into one image, at the moment, I do it through 
checking their pixels' dimension, and then creating a big new image, and
then copy and paste onto the new image

It is okay but it is hard to align them nicely with a mouse and 
calculating pixels dimension isn't enjoyable, especially when there are a lot of images to create

is there any wiser way to have a lot of images to be combined into one image?

should be done through GIMP