First of all, ensure that you have a backup of all the data on your hard disk. Playing around with partitions can be dangerous. Just follow the exact steps I mention, and you should be fine

Second, I noticed that you have used 3GB from sda12, which is the partition named "Linux". We will be deleting this partition, and everything in it will be gone. If you need the data on it, please take a copy of it before proceeding. This partition is not your Ubuntu partition, it's just the partition that you see in Windows called Linux.

Third, in Linux, there's a partition that you can create called swap. This partition is optional, but is almost always needed. The purpose of this partition (without getting into detail) is to make Ubuntu work faster, so the following steps will include this partition. We'll be making it 4GB. So in the end, your Ubuntu partition will be 11GB out of that 15GB. You can skip it (which is step 2 in the steps below), although I advise you don't.

Fourth, in the end, when we start the process, it might take a long time to finish. Resizing and moving partitions usually takes time. In your case however, your partitions aren't that large, but still, it might take 1-2 hours or something. If this is a laptop, ensure that it is plugged into a power source.

Fifth, and finally, read the whole answer before doing anything.

Okay, you first need to boot into live Ubuntu. Boot from a bootable USB/CD just like if you were going to install Ubuntu, but choose "Try Ubuntu" not "Install Ubuntu". Open GParted, and follow the steps below:

1. Right click on /dev/sda12 > Delete


2. Right click on the now unallocated space > New
   
   
   
   1. In New size (MiB), type 4096
   
   
   2. Make sure Free space preceding (MiB) is 0 (it's okay if it's 1, or some small number, and you can't change it)
   
   
   3. Change File System to be linux-swap
   
   
   4. Click Add (or OK, or whatever the button is)
   
   
   5. Now, you should see a swap partition and unallocated under it
   
   


3. Right click on /dev/sda16 > Resize/Move


4. See the bar on top in this resize window? We need to "expand" it to the left, so drag the left end of it (the little left black arrow) to the left until the end, and click OK (or Resize, or whatever). You should now not see any unallocated space.


5. Right click on /dev/sda17 > Delete


6. Right click on /dev/sda16 > Resize/Move


7. We'll do the same thing in step 4, but expand it to the right. So drag the right arrow to the end, and click OK.


8. Everything that we've done actually hasn't happened yet. GParted is just recording these "actions", and now we need to apply them. But before we apply them, take a look at your partitions now. You should find that you now have a linux-swap partition (4GB) and under it your Ubuntu sda16 partition (around 11GB). If this is not the setup that you see, then please close GParted and restart from the beginning. (If there is unallocated space of 1MB or something like that, you can just ignore it.)


9. Once you have checked, double-checked, triple-checked, hit the green "check mark" in GParted to apply all operations.

Alternatively

Do you need your current installation of Ubuntu, or can you delete it and reinstall it? This will make things much easier. If you can delete it, just do this:

In GParted, right click and delete sda12, sda16 and sda17, this will leave a big 15GB unallocated space. Apply the changes in GParted. Now, start the Ubuntu installation. When presented with options, choose "Install Ubuntu alongside Windows XP". This will install Ubuntu into that 15GB space automatically (it will take care of creating the swap partition and all that).

Okay, after the comments, do this:

1. Delete the unknown partition


2. Right click and resize your Ubuntu partition sda14. Give it a new size of 16GB (16384 MiB), and make sure that the free space is on the right (don't worry, you shouldn't see the error message about booting problems. If you see it, just ignore it, this time it doesn't really matter because we didn't "move" the Ubuntu partition).


3. Now, you should see around 4GB of unallocated space. Right click it > New, and choose file system as linux-swap, and hit OK.


4. That's it. Take a look at the partition table. It should be like the screenshot you added in the comments, but sda14 as 16GB and linux-swap after it as 4GB. Apply. Reboot.