Disclaimer: I'm not an artist so this is just programmer's art knowledge.
You're having the grid effect in your example mostly because of that lighter patch of grass on the bottom edge of the tile:

Details like that that are easily recognizable instantly give it away that you're just repeating the same tile.
Check this article which has a lot of useful tips on the subject. In particular, using Photoshop:
- Use the Patch Tool to get rid of glaring details.
- Use the Doge or Burn tool to get uniform luminosity everywhere.
- Make the edges seamless by offseting the image and then using the Patch Tool to blend the edges together. I've also seen people duplicate and mirror the image four times before for this purpose.
But more than that,Also in the image you don't seemlinked there seems to be rendering your tiles correctly either, sincean implementation problem too because when zooming in on the image you can clearly see thesome gaps between tiles:
