Making a picture tileable

This tutorial will teach you how to make a picture into a tileable one. All you need is a 2d picture editing program (e.g. Photoshop, Paint Shop Pro, Painter..). A graphic tablet isn't required, but very helpful.
The main purpose of doing this is that you have a pic you want to use as a texture in a 3d program (such as 3D Studio Max), but the picture isn't as high resolution as you'd like, thus you have to repeat it more times. If the picture isn't tileable, you will clearly see the borders where the two copies meet.

I will use a cloudscape photo I want to make horizontally tileable. If you need a pic that is vertically tileable, proceed the same way, but edit the top and bottom borders.
All example pictures are scaled down to 50% of the work resolution, except the original and final version.

Step 1. Extending the borders

Extend the right (or left - it doesn't matter) border by a specific amount of pixels. This amount depends on the type of your picture. If the picture contains big objects or patterns, use more, approx. the size of one object (it will also take longer to finish).
I choose the right border and extend the pic by 100 pixels:

Step 2. Copying the tile

Copy a part of the picture, opposite to the border you extended. The width of the copied area must be the same as the amount you used to extend the border.
In my pic, I selected the left 100 pixels wide part and cut it. It doesn't matter whether you copy or cut it - you won't need the left part anymore:

Step 3. Closing the picture

Reduce the width of your canvas by the specified amount of pixels, so that you get rid of the part you don't need (the white area in my picture).
I reduce the left part of the canvas by 100 pixels. Now, the size of the picture is exactly the same as it was when beginning, but you can clearly see the seam between the borders of the original picture.

Step 4. Removing the seam

This is the last step and the longest. Basically, you edit the picture by hand and try to make the seam invisible. The only rule is that you mustn't change the horizontal borders. They "connect" to each other (if you tile the pic now you won't see them). I can't give you an exact description, it depends on the picture but there are a few tips you mind keep in mind:

• Use the colors from the rest of the picture, not new ones because that probably will be visible.
• Copying tools are very handy when the rest of the picture has a fine texture that is difficult to reproduce. In Photoshop, this is the Clone stamp tool.
• Look where light is coming from and try to shade the added objects the same way.

In the clouds picture, I looked at the other clouds and copied parts of them and edited manually to create new ones that fit into the picture. I think this also teaches you how to draw the type of that object as you're forced to study it very carefully.

Here's the final pic reduced to 50% and repeated in horizontal direction:

If you need to make a picture tileable in both directions (horizontal and vertical at the same time), do it first for one direction and then for the other one. This is somewhat difficult to make it perfect though, some small errors may appear in the corners of the picture.