Is there any way I can undo terminal command "killall Dock?"

Hello,

I just bought my new iMac yesterday. Last night I set up a new desktop background - a picture of my dog - this morning it's gone. I tried to google why and came across a person telling me to enter "killall Dock" in my terminal.

So I did that and later, I come across another reason as to why the background is gone - I simply copy & pasted the photo from a thumb drive & didn't save the photo to my computer.

Now, my desktop background is simply blue - the background won't change to the picture I want. I tried going into system preferences - it shows the picture of my dog as the background, but it doesn't show up.

I restarted my computer to no use.

Please advise. Should I somehow reverse the terminal command or...? Did the terminal command to any damage to my new computer?

Thank you for any help.