I have tried one of the creative sites and downloaded an app but had some trouble and couldn't get a reply from the site. Thanks for you encouragement.
I can remember this being asked on this forum before, but I can't find that thread now. There are lots of references to using Photoshop to create cross stitch and lots of videos, but I wouldn't like to judge how effective they are.
This link has the advantage of coming from a craft site, which should give it some credence.