First tut, so be gentle on me.
Alright, at first you need the pictures you'd like to turn into a brush pipe of course. For my example I'll use just two, but as far as I know there isn't a limit for the number of brush elements, so you can turn even ten or more pictures into a brush pipe.
Step 1: Open GIMP and create a new transparent image (The size loosely depends on the size of the brush elements. What matters is that they all fit into the new image!).
Step 2: Open your first brush element as a layer by "Open as layer" -> "[path to image]".
In most cases. the brush elements need to have alpha channel though, If your pictures are .jpegs or any other format that doesn't support alpha channels, you'll have to add alpha manually. Go to "Layer" -> "Transparency" -> "Add alpha channel" and then take away everything that doesn't belong to the actual image until there is just the element you want on a transparent background. (See example above)
After that, delete the very first layer (the one you created at start-up).
Step 3: Open the next image - again - as a layer and place it as a new layer in the same position right above the previous one (that should actually work automatically.
Step 4: Repeat step 3 until you have all the images you want for the brush pipe as separate layers placed above one another.
Step 5: Cut the image down to the smallest size in which all the layers fit into, by "Image" -> "Cut down automatically".
Step 6: Save your file as [filename].gih (gih stands for GIMP image hose)
A window will appear:
In the second line you can insert a custom name for your pipe. Careful: It's not the file name that determines the name of the actual brush pipe. If you just leave it be, your pipe will appear as "GIMP Brush Pipe" in the list of brushes, which is quite uncomfortable if you have more than one pipe later on.
The other settings you can just adopt as they stand except for the "rank". Up the value for rank to match with the number of different layers you have used for the pipe. In my example I would insert the number "2", because I used only 2 layers.
Step 7: Hit on "Save" and put the new file into your brush folder.
That's about it. Now you only need to either reload your brushes or close and re-open GIMP and you can paint with your brand new brush pipe.
Hope this tutorial was understandable! If I have blundered with the correct translation (or anything else), any moderator may feel free to edit this post.