When making changes to a file from within WinMerge it will create a backup file based on the original filename and then .bak appended. This is usually a silent process.
Today I first ran into an issue with a file name that starts with a . where this scheme fails.

Since images cannot be searched as easily: the message says that the backup could not be created and mentions "C:\Users\oschneider\.bashrc (-> C:\Users\oschneider.bashrc.bak)". Note the missing \.