Using photo reference for art is important and good. Selling work that uses photo reference (where the photo is taken by someone else) is not bad as long as your work isn't identical to the original.
Tracing is not bad. It's actually a really good learning tool. But tracing work that's copyrighted to someone else and then selling it is very morally iffy. If someone found a drawing you made and traced it to learn how to draw a thing, that would be fine. If they tried to sell a trace of your drawing, that would be unfair to you.
I've traced before to help when I had to draw something I was very unfamiliar with in a short amount of time, but I made absolutely sure we had rights to the photograph first. Photographers have the same rights to their work that artists do. It's entirely possible that the free wallpaper site you checked didn't have rights to the photo, either; they just saw a cool photo and put it on their site.
So, I think the question is, "Did you just reference the photo, looking at it while you worked? Or did you trace the photo, so that all the lines will match up if you put them on top of each other?" I think it makes a difference! And it might be something to keep in mind for stuff you make in the future!
You're not going to go to court. If you found this on a free wallpaper site, the photographer is not gonna be fiercely hunting down copyright infringers on this picture. It's more a matter of what's morally right, and a lot of people don't feel good about artists selling work they traced from somewhere else.
As far as the actual post, I know it feels awful and sickening to come across something like this -- just one comment like this is hard to take, much less 20 -- but 20 people on the internet is an amount you can ignore, honestly. If someone posts "lol look this artist traced a thing & they're selling it" to a blog that's specifically geared towards gossipping about artists, then everyone who reads that blog is going to be naturally inclined to agree with the poster. They're not invested in your particular case and I doubt they'll "destroy you;" they're just agreeing with their little gossip circle.
Basically, someone tried to start hate on you, a few people went "ew tracing is bad," and a couple other people responded "well her webcomic is good though actually." I think .... that's actually really cool??? This person's internet hate couldn't even make it past 6 comments before people were stepping in to point out that you're a good artist. THAT'S SOME PRETTY INEFFECTIVE INTERNET HATE
TL;DR -- working with reference is fine and good and you don't need rights to the photo to do it! But it's best to have more than one reference so you don't match any one photo too closely.
Tracing a photo is a fine way to learn, but most people feel it's wrong if you trace someone else's work and try to sell it.
But, even though it's normal to feel really horrible after running into this kinda talk, this tiny group of 6 people trash-talking isn't as big a deal as it probably feels like it is!! That's not the tumblr army, and if they're not reaching out to contact you, I think they can be safely ignored.
