The preview.jpg gets updated every time the map saved, and it does save when you publish the map, but it's possible to "force" a custom image.

- Save your map in the state that you want to publish / update it.
- Go to \Users\UserName\AppData\LocalLow\Team17 Digital Ltd\Golf With Your Friends\CustomLevels\MapName
- Replace the preview.jpg with the image file you want, it should be 800x800 iirc and be a .jpg as well, set it to read-only.
- Back in the editor publish your map. It will throw an error message because it can't save the new preview image, simply click through that and it should now publish the map with your image.
- Either remove the read only flag on the file or rename it to something else, otherwise you won't be able to save changes to the map later.

this works when we already posted the map or?
bc i followed all the steps and its not working

It should when you publish an update to an already posted map, but it's been a while since I actually done it. This post is almost three years after all. :)

Might be possible that the order of steps isn't quite that anymore if T17 have changed something abut how the "in game" side of pushing content to the workshop is handled.