@burnout426

https://addons.opera.com/en/wallpapers/details/finding-new-ways-nathan-anderson/

It changed pictures that I selected from my pc, from easy-setup, from addon site...
I supposed that the reason may be one of the extensions, so I checked all. One of extensions didn't work at all. Nimbus screen-shot. It didn't respond to any click. So I removed it. And since then wallpaper hasn't been changed to default yet.
Could the reason be in one non-working extension?

I have a feeling that Opera support hides something.
Please explain me one thing. If all these files are in my Opera Themes folder, why I can't see them in my Opera settings. Is this the folder containing our wallpapers? If I have already these zip files in "themes" folder, how can I use them? Unzip and then set the picture as wallpaper? Doesn't that sound like something stupid?

If "themes" folder contains all our wallpapers, so where Opera takes all these awful wallpapers from?

These Wallpaper 1, Wallpaper 2... Because I replaced the picture in "default_dark_theme.zip", but this raunchy picture with circles is still set as my wallpaper. And it's still there if I'm offline, so Opera takes it from one of my pc folders.

It changed pictures that I selected from my pc, from easy-setup, from addon site...
I supposed that the reason may be one of the extensions, so I checked all. One of extensions didn't work at all. Nimbus screen-shot. It didn't respond to any click. So I removed it. And since then wallpaper hasn't been changed to default yet.
Could the reason be in one non-working extension?

@leocg is it the answer to all our questions. To name a folder that we always knew and that contain only two default wallpapers. I have already told that these wallpapers in theme_backup are SVG but others in themes are JPG. May be that is the reason. I tried to replace them, made another svg file and replaced that reborn3_dark.svg with my picture. But surprisingly Opera ignored it and my wallpaper again was that picture with awful rings. So I assumed that Opera has another folder for wallpapers. I did everything with that files in themes_backup, but Opera restores them after all, even if I'm offline. How?

When I see such answers of support and moderators, which completely do not answer the questions of users, I understand that it means - "f--- off, we don't know the reason of the bug and can't tell what to do". Ok, no problem.

@burnout426 off course it's not because of one picture. There was no need to check, I mentioned that Opera changes any picture. I set as wallpaper a picture from a folder on desktop, and surprisingly it has changed only once since then. But if I choose a pic from Opera stable folder it will change many times a day. Can't understand the mystique Opera.

So what about the real place of Opera wallpapers? Does anybody know? Is it so secret?

I have a feeling that Opera support hides something.
Please explain me one thing. If all these files are in my Opera Themes folder, why I can't see them in my Opera settings. Is this the folder containing our wallpapers? If I have already these zip files in "themes" folder, how can I use them? Unzip and then set the picture as wallpaper? Doesn't that sound like something stupid?

If "themes" folder contains all our wallpapers, so where Opera takes all these awful wallpapers from?

These Wallpaper 1, Wallpaper 2... Because I replaced the picture in "default_dark_theme.zip", but this raunchy picture with circles is still set as my wallpaper. And it's still there if I'm offline, so Opera takes it from one of my pc folders.

I unzipped this picture from the theme's zip file, cause I thought the reason is in wrong zip.

But it doesn't matter. Opera resets ANY picture file to default dark theme wallpaper.
Several times a day. But this time took longer. I can't understand the algorithm. )))

I think maybe the reason is that the default wallpapers are SVG.
"reborn3_dark.svg" in "default_dark_theme.zip"
But all other wallpapers that we choose are JPG.

Ok. I have an idea. I'll rename one of my favorite pictures to "reborn3_dark", change the type from jpg to svg and will replace the original reborn3_dark.svg in default_dark_theme.zip

So if Opera decides to change the picture again, it will change to picture I want.
Will it work?

@burnout426

https://addons.opera.com/en/wallpapers/details/finding-new-ways-nathan-anderson/

I forgot to mention, if I install a wallpaper (for example this one https://addons.opera.com/en/wallpapers/details/finding-new-ways-nathan-anderson), after Opera change it to default, I have to install it again and again.

Really?
And what about these people?
https://forums.opera.com/topic/26089/wallpaper-resets-between-sessions
https://forums.opera.com/topic/22737/wallpaper-resets-between-sessions
https://forums.opera.com/topic/24393/wallpaper-keeps-disappearing
https://forums.opera.com/topic/29733/trouble-with-speed-dials-wallpaper
And many others.
If you search in google then you'll find out so many questions about Opera wallpapers.
It's funny, but why Opera team can't make this simple option work as it should?

Opera resets wallpaper occasionally. And it does not depend on restarting the browser or sessions. For example the browser was opened on Speed Dial and I was doing nothing and suddenly it changed the wallpaper to default wallpaper 1.
This happens only to pictures (static wallpapers), animated wallpapers are not changed.
I have posted already about it but got no answer, and noticed that many people already have the same problem, but there is no any clear answer from support. Only questions to question.

The wallpaper resets occasionally from time to time. And it does not depend on restarting the browser or sessions. For example the browser was opened on Speed Dial and I was doing nothing and suddenly it changed the wallpaper to default (Wallpaper 1, which I hate).
This happens only to pics (static wallpapers), animated wallpapers are ok. Why it should change from Wallpaper 30 to Wallpaper 1 by itself?
And how can I delete that stupid Wallpaper 1?