What could it mean, when your iMac wakes up from sleep mode, and you have an abstract desktop picture that you did not set?
I have encountered this somewhat lately. I have all the updates on my iMac, and I'm pretty tight on security. My hard drive crashed in June, and I had to replace it. My iMac is 4 years old this month. Otherwise, everything seems okay. Could this be warning signs of things to come? Any ideas?
iMac, macOS High Sierra (10.13.1), null