Well, how about your graphics settings? Those have a large influence.
Same with any graphics card personalization, like I have a slider for more performance or quality.
my graphics settings are for high on some, the problem is the way jpeg screws over the image quality, no matter what, try making a pic in a image program and save first in png then in jpeg, jpeg often changes the picture to make it a bad quality, at least for me
edit: as for the general look themselves, i have gotten alot of help, so if i use steam screenshots, i get a better picture quality usually

but gmod tends to crash for me alot, so i take screenshots of progress often