I want to change the picture on my IMAC when I first power up the IMAC

If you're talking about the profile photo on the login page of your iMac, you can easily change that in the iMac's System Preferences. From there, go to Users & Groups > Click on the Password tab > Hover over the current profile picture, then click on "Edit". From there, you can change your profile picture to your liking.

If you'd like to change the background picture of your iMac, you can do so at System Preferences > Desktop and Screen Saver > Click on the Desktop tab, and from there you can choose between any of Apple's stock photos or solid color backgrounds, or you can choose your own photo from the Photos app.

I hope this helps!

Maybe this?

https://www.macworld.com/article/3128967/how-to-change-your-login-screen-in-macos-and-change-it-back.html

Or the older...

https://computers.tutsplus.com/tutorials/how-to-customise-the-background-image-on-the-mac-login-screen--cms-27625