If you're looking for something a bit more advanced than Microsoft's Paint but not as complicated as the GIMP, then try Paint.NET - those with a Photoshop background will have no problem and it's a bit easier to pick up than the GIMP.
I also recommend Irfanview and Faststone Image Viewer.
Finally, for checking photo rotation, use JPEG Lossless Rotator. You can rotate photos and also reset the EXIF rotation. Very handy, even on systems like Windows 10 that try to manage the EXIF rotation metadata.
