Ok heres what you can do.
First crop the 512x512 section from the original photo just like you did. But if you use it just like that all the details will look too big, and you have to scale it really low in game, just like happened in this case.
So what you do is after croping you encrease the canvas size (not the image size) to 1024x1024 (or 2048x2048 if you're feeling like it
). Then place the 512x512 section on one of the canvas corners (lets say top left) and copy that section 3 times, and place each of those copies on a different corner of the image.
So now you have the same 512x512 section repeated four times on a 1024x1024 image.
You then have to make it tile, off course you need to do that twice, and don't forget to merge all the layers before starting doing it.
After finishing it, in case its still too obvious that you have four sections repeated on the texture just pick pieces from other photo sources and blend them with it to give some randomness.
Now once you're finished scale the final product to 512x512, you should do this only once the texture is 100% finished so that you dont loose detail. Now you have a better scale to use in game, and hopefully some of the blur will be gone giving it a sharper look. If that doesn't work run a Unsharp Mask filter and play around with the settings untill it looks better. Don't over do it tough.
In case this texture is going to cover a large area its better if you do it 1024x1024, so that it will be harder to tell where it tiles. In that case you need to copy the section 7 times, and encrease the canvas to 2046x2048 and finally scale it to 1024x1024.
I also advise you to play a bit with the brightness/contrast and hue/saturation options to give it more color and contrast.
Hope you can understand everything and that this helps.