Gimp-Forum.net

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

You could try the gmic plugin http://www.gmic.eu Use the filter Arrays & Tiles -> Montage Four image is easy: More than four can be tricky.

Load the images as layers, then the filter settings depend on if the images are the same size or different sizes Order depends on layer position.
same size, merging mode aligned: https://i.imgur.com/RHCO83U.jpg
different size, merging mode scaled: Make sure you load the largest image first to avoid cropping: https://i.imgur.com/rRWdt3Z.jpg

another example: 6 images, different sizes: https://i.imgur.com/plS44Yi.jpg

edit: further example 20 images, resized to (roughly) same size using BIMP and montage Vertical Array setting https://i.imgur.com/c5o2dYH.jpg