Open the photo in GIMP.
Go to Colors > Levels to open the Levels dialog.
Press Pick, which looks like a pipette with a gray stem.
Press somewhere on the photo using the gray point picker to define what is a mid-gray tone. ...
When the colors look natural, press OK.
Well GIMP is just fine for what it is a free photo editing program.
I use GIMP, and my XP-Pen Star 06 Wireless Tablet works almost perfectly with it .