I figured it out!
It wasn't the inappropriate image at all, but simply just my desktop wallpaper (outer space) that looked weird in the small thumbnail! and the reason it lagged so much when it was visible was that the image has a resolution of 25000x5208. (which opera understandably didn't like) Also, I had no idea my wallpaper was that big!

I deleted the contents of the temp, %temp%, and the wallpaper folders.
It didn't work.

I'll try and uninstall opera and delete everything opera related.

I accidentally clicked "use image as wallpaper" on a somewhat inappropriate image while in a private window, it didn't change the theme, but placed the image in the "easy setup" menu under all wallpapers. Whenever the image is visible on screen, opera slows down to a crawl. I cannot remove it as the x doesn't appear, and I cannot change to it.
The image doesn't appear in settings, which I find a bit odd.
I even tried to reinstall opera, (i checked the delete all user data box) but it didn't work.
Does anybody know where opera stores wallpaper images, or how i can fix this?

If i tab out (Windows button + D) from Youtube when in fullscreen, it freezes with usually most of the screen white in fullscreen, and in the corner you can see a bit of the video that was playing. It is completely unresponsive, but i have full access to windows so i can close the window from the taskbar.