USEFUL DATABASES LINKS:
1 - Kodak Lossless True Color Image Suite: a database commonly used in papers for evaluation purposes. Consists of 24 images. The images were never compressed. The files are 24-bits lossless PNG. LINK: http://r0k.us/graphics/kodak/
2 - Tecnick Image Set (TESTIMAGES): a collection of sample images designed for analysis and quality assessment and image processing techniques. The archive has more than 2 million imaves divided in different categories. For example, a paper has used the SAMPLING category, RGB, 1200 of size. LINK: https://testimages.org/