Mac LOCKSCREEN Trouble

When you attempt to alter a filename extension macOS will generally warn you like so:

... but you can insist by clicking Use .heic or whatever.

What file are you attempting to rename and where did it come from?

Interesting. Try dragging that file to your Mac's Desktop, rename the file to Mojave.heic there, and then drag it into the folder where the original Mojave.heic resides. You will need to authenticate—a requirement for making any change to that folder.

As I recall that's what I did... no particular magic required.

Be sure you archived the existing Mojave.heic file as I wrote, or select "keep both" when the Finder asks you to resolve the duplicate file name.

FYI these are my Finder Preferences:

Change the ones regarding filename extensions to match them, at least temporarily. It might be related.

I have no doubt it's not supported. Whatever you do is also likely be undone upon the very next macOS update. I'm at a loss to explain why it's not working for you.

I suspect there is something I am not explaining well but I don't know what it could be.

Just to be sure it wasn't something related to Photos I also tried dragging this photo exported from my Photos Library and renamed it:

No problem.

The Mojave login screen background image is /Library/Desktop Pictures/Mojave.heic. You can change it.

To navigate to the image file directly, copy and paste the above bold text in the Finder's Go menu > Go to Folder... field and click the Go button.

A Finder window will open with that file already selected. You can change it to whatever you want, but its name still has to be Mojave.heic and its dimensions have to be appropriate for your Mac's screen resolution.

I recommend creating a duplicate of the existing file first so that you can undo whatever you do.

When attempting to rename the file, are you selecting the whole file name? If you just click the name once, it will not select the extension. You have to do that yourself. Drag or triple-click the file name.

Changing the file extension does not convert the image. I wonder if that is the problem.

I will test later when I have time on my mac, but I suggest opening the desired image in Preview and doing Save As... choosing the format as HEIC

That's a good thought but I did not need to convert the image or change its file format. All that was necessary was to rename an image (a jpg, in my example) to Mojave.heic and put it in place of the original. macOS didn't care.

Although I do not know the reason Apple chose that particular file format, it's not a requirement for the OP's purposes. It and Solar Gradients.heic are the only ones in that folder using that format. That's also the reason I think the modification won't stick after a macOS update, and the reason Apple said changing it wasn't supported.

What happens when you try? You need to authenticate but that should be all that's required.

Here's Ink Cloud.jpg in the same folder, simply renamed to Mojave.heic

I went and tried, and yes, even if I take a jpeg file and rename it as Mojave.heic, it works.