If you can’t open your ArtRage Touch painting files, they are probably too large for the app. There is an issue with some devices with larger screen resolutions overriding the canvas size defaults – basically, they have created paintings too large for the app to re-open.
If you have a licence for a desktop version of ArtRage, then just install that on your Surface Pro (or other device), open them, and change them to be 2048×2048 pixels or less and it should solve it. The free demo will work as well, but it will save the files as much smaller paintings (maximum size is 1280×1024).
If you keep an eye on the canvas size when creating new paintings, you can manually change it down to 2048 pixels or less for width and height and avoid this problem.