Hello.
When we make a forum or a website where you put a mosaic-like pattern as background image, the most common problem is the fact of having a cutline between two repetitions of the pattern.
With this tutorial, I will show you how to create your own patterns without having these horrible cutlines.
Here we go !
Open a new 400*400 pixel document (File > New)
Once the document created, choose the color who will be the pattern's background. For example, I choose the color 9fdcff who is a soft lightblue. Fill in the whole background with this color using the bucket tool or doing Edit > Fill with FG color or BG color (as long as it's the blue one).

Now, create a new transparent layer called "Brush"
Now, take the Paintbrush tool and choose a personalized brush who's smaller than the document (if you haven't installed any, look in my "install brushes" tutorial).
Then, choose a darker or lighter color for your brush. On my side, I chose white.
Apply your brush in the middle of your pic.

You can then play with layer modes, feel free to do it or not

On my side, I set it as "Grain Extract".

Duplicate your layer. We'll now offset it.
Go to Layers > Transform > Offset
As offset value, set the half of the image's total dimensions in both fields (on my side, I put 200px, since my pic is 400*400 big). Validate.
Result :

You can also offset it only vertically or horizontally, or even combine them all.

Here are the variation I have done.
Horizontal+vertical

Horizontal

Vertical

All of them (in this case, duplicate the layer three time and apply them a different offset)

I know, the result isn't extraordinary, but you can choose the dimensions, colors and brushes you want since this technique leaves a large place to creativity.

Now, all is left is to save your document under .png

Save a .xcf copy if you think you will work on it later.