The genome_coverage_quotes.png tells you how how many % of your genome are covered by at least x-fold. If you want to calculate an average value then see whether there is a text file in qualimap that allows to get the values from that graph and calculate a mean. Looks like roughly 90% are covered at least 20x, 55% are 30x, 25% are 40x, will probably average out at something between 20x and 30x.