what happen if client diff it to read data folder first?
If you allow your client to read the data folder first, you'll be able to see it on your side first. Because, I believe, you're "testing" to see if the sprite shows and by doing so you're doing it with your data folder. Thus, if your client is not diffed to read the data folder first, you will not be able to see the sprite being changed. Furthermore, with this in mind, diffing your client to read data folder first will allow you to make any edits to your data folder and your client will be able to read it, thus allowing you to see your changes without having to patch your grf.
I see.. the data folder name must be the same as grf?