You can make seamless textures using gimp. It's an open source (free) photo editing software. gimp.org. 
 
1. Open the image file in gimp.  
2. Edit > Copy, then Edit > Paste as new layer. Paste it as a new layer three times so you have a total of four copies, each on their own layer.
3. Image > Canvas Size. Exactly double the width and height of the image/canvas size.
4. Use the alignment tool to align each layer in its own corner (so they are all visible, not overlapped).
5. Use the "flip" tool to reverse/mirror three of the layers. Leave the top-left corner alone. Reverse the top-right corner horizontally. Reverse the lower-left corner vertically. Reverse the lower right corner horizontally and vertically.
6. Export the new image. This will now be a seamless texture when tiled. Choose it as the texture file for your material.
7. Adjust the scale of your new material since the size of the image is now larger.
 
You can do it in photoshop, but I can't give you the step-by-step since I don't use that program.