"join two images" is a quite vague description, but I'm sure you can do what you want with Gimp. You can install it using Software Center or from the command line using

sudo apt-get install gimp

It is probably closer in functionality of Photoshop than to MSPaint though... which is a good thing.

Here's how to do it in GIMP:

1. File > New ; create image bigger than both of your images to join combined.


2. File > Open as Layers ; open your images.


3. Use the Move [M] tool to arrange your images.


4. Use the Crop [Shift+C] tool to crop everything when finished rearranging.


5. File > Export to... to save your output file.

That's it! Instead of GIMP you can use Pinta