Hi Blue Poppy

Do you have any pictures that are definitely of your grandad? Why not post one and we'll see if we can compare it to the one of the soldier, to see if there are any similarities?
Just to confirm Jim's post, yes this is a photo of a WW1 soldier

If you were to post a bigger, higher-resolution image, we could get the badge identified. If you can find an existing service record for your grandfather, this could be compared to the cap badge ID.
Cheers
Prue