Terraformed Mars from human base on Phobos by David A. Hardy

In 1994 the famous British space artist David A. Hardy created several retrofuturistic illustrations of a human base on Phobos, the closest of two natural Martian moons, in front of a terraformed Mars. The illustrations were created as cover art for two separate books. More of Hardy's Mars art here.

"Terraforming Mars" as cover art for Arthur C. Clarke's book "The Snows of Olympus" (1994):

"Terraformed Mars" as cover art (there are several slightly different versions of this illustration) for Carl Sagan's book "Pale Blue Dot: A Vision of the Human Future in Space" (1994):

Mars colony illustrations by Luis Peres

Nice illustrations of a human colony on Mars by Portuguese fantasy artist Luis Peres for "Generation Mars: Book One - Scratching the Surface" by Douglas Meredith. The goal of this book (published on August 2019) and its upcoming series is to provide younger readers with more serious and realistic science fiction that still tackles the main “what if” quality of the genre. It gives a glimpse into what life on Mars might be like for kids just like them.

The main character, Cas, is the first human child ever born on Mars. Martian kids are living what is, to them, ordinary lives in their underground colony, but for the first time they will be allowed to see and explore the surface of their homeworld. The details of the habitat, its life-support systems, and surface suits are rooted in realistic science and technology, with further details explained in a postscript to the book. Future books of the series will address fundamental issues of survival on Mars: air, shelter, water, food, etc.

Human colony on Mars for Buzz Aldrin's "Welcome to Mars" book

Impressive cover art of human colony on Mars with several domes and parks for "Welcome to Mars: Making a Home on the Red Planet" book for kids. It was written by Marianne J. Dyson and Apollo 11 astronaut Buzz Aldrin (2nd on the Moon) and published in 2015 by National Geographic Children's Books.

Alien Martian canal by Mark Salwowski

Picture of the Day 11/10/2018 - Alien Martian canal and a city by Australian illustrator Mark Salwowski - a cover image for Lance Parkin's and Mark Clapham's book "Beige Planet Mars" (1998).

Astronauts watching a spaceship landing on Mars by Stephen Youll

Picture of the Day 10/6/2018 - astronauts watching a spaceship landing near a canyon on Mars - a cover image by Stephen Youll for Gregory Benford's book "The Martian Race" (1999).