It's difficult to get the perfect lighting within photos taken by cameras, this depends on many factors that may cause your photos to look darker than usual or too bright. An easy solution to correct bright or dark pictures is to use third-party applications to adjust the brightness or improve lighting of your photos to get good quality.
If you are a photographer running Ubuntu or Linux Mint, you can use Urban Lightscape to help you improve and harness your photos before printing them. With Urban Lightscape, you can correct exposure, improve lighting, add synthetic light to your pictures, and make localised brightness adjustments. The user interface of the application is very simple to use and allows to save manipulated images in many image formats such as bmp, png, jpg, tif, pnm, pcx, xpm, etc.
sudo add-apt-repository ppa:dhor/myway
sudo apt-get update
sudo apt-get install urbanlightscape
Urban Lightscape Home Page