This Is Not A Photography Site
Highlighting that geograph is not primarily about 'artistic' photography, preferring to concentrate on documenting the land, warts and all. Sometimes that means excellent photography, sometimes not - but first and foremost we welcome contributions from photographers whatever the skill level.
It depends on how closely your painting resembles the photograph. I can't see any problem with using the information gleaned from any Geograph picture to inspire your painting. On the other hand, if you were to trace the photo and paint it over, you should credit the photographer for their contribution.
If you are in any doubt, just email the contributor (there's a link under the main picture) and ask them; I'm sure most will answer in the same way. You could offer to take a photo of your finished painting and send it to them as a thank you - many would appreciate that.
Geograph aims to document geographical features and locations across the British Isles. While we appreciate good photographic quality, our focus is more on documentary value rather than artistic masterpieces. We look for images that are in focus and well-exposed.
Our "mainstream" images show geographical features and the location of a grid square from within that square, at a reasonable range. However, we also welcome:
Close-up and detailed images: These can add extra interest and context.
Indoor or underground images: While not our primary focus, they can help in understanding a square.
Long-range images: Photos taken from mountaintops, drones, planes, or other high-altitude cameras are very welcome.