after the I received update from appcenter, when I import pictures to wallpaper they are not shown up as thumbnails. Only blank. Oh it's eos 5.1. Anyway to fix it? And where do I report the bug?
Sometimes I have the same problem, but when I switch the system settings to the main tab and return to Background - there is a tumbnail image. Try this, if it do not work, you can post a bug (called an issue there) into https://www.github.com/elementary - just find the appropriate section and follow the rules for posting issues in order the team to take a look on it.
Edit: I just found out that you are using a different theme then the original. Before you file an issue, check out do you have the same problem with the default theme - sometimes the problems came from using not well written themes.