Not logos used for Linux are penguins that are black with wording saying linux. I would think you can use any off google as long as who ever uploaded it allows it. Search google for free linux logos and I'm pretty sure tons will show up.
Tux is the "general" linux logo/mascot/symbol where then each Linux distro has their own logo. Use Tux to give general linux usage, and then the distro logo for specifics.
yaa... its Tux that comes into my mind when linux logo is asked.
These days, companies prefer to keep Linux OS logo like CentOS, RedHat, CloudLinux etc.
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actually i redid it a bit ago a dif way for several reasons, because what i thought was a dot at the end of redhats logo is actually the register symbol so i had to redo it anyway so i changed it, with the reg symbol i dont need to use the tm on redhat, plus i very slightly changed the text color on linux so it would show up better, i just didnt have time to get back to post a new image...
I did not correct this part, after looking at several sites for linux i did notice that some of the images actually intentionally showed the outward glow around tux, some of the images look like they were just shadows left from copy and paste, but some of them actually look like an intentional glow around the body. I spent quite a bit of time removing much of that but that was before i noticed the other images online with the glow. So i will leave it off, i dont know if it was intentional by linux or not to have that glow, if i find out that it is, i guess i will put it back or start over and leave it..
here is the newist version. And also i have included a nice redhat logo i got off their site, it looks good on my blue background.. you can see it but it says redhat on it, maybe its better it if you dl it not sure.