For anyone who may have ran into error code 25 when attempting to upload your game to the workshop: I released a patch yesterday that fixes the issue, so it should work if you try uploading again! The issue was related to the file size of screenshots exceeding Steam's limit. Special thanks to community member Wrinkly for reporting this issue!
Here's the Steam Workshop thread for anyone interested:
https://steamcommunity.com/workshop/discussions/18446744073709551615/597404744772811132/?appid=498310
