I am running OH on Windows 10. The original file is being copied off the internet using wget, to a temp name. Then I copy that file using a batch file with the standard copy command to the well-known name that I'm referring to in my sitemap. If OH is shutdown, I can copy over my image without any issues. When OH launches, if I do not go to the UI page that shows the image, I can copy over the image file without issue. As soon as I go to the UI page where I show the image I am no longer able to copy over the file. Even if I exit that page and go to other pages, it is still locked.
I have used sysinternals Process Explorer to see what app has the file open and it shows java.exe.
I'm doing a similar file overwrite with my BlueIris app for cameras, to multiple static camera images. They get refreshed without any issues, even after I've viewed them in the oh UI.
I'm running OH 1.8.1