The "\" character in Python is a string escape, and it introduces shortcuts for certain string characters. For example the string "\n" doesn't contain the characters \ and n. It contains a newline character. Windows paths always cause this trouble in Python. When Python sees "\U", it's looking for some unicode escape that doesn't exist.
You can use raw strings in Python by prepending the string with r.
searchFolder = r"C:\Users\rohrl\OneDrive\Python\PictureCompare\MixedPictures"
Or you can get in the habit of using double \\. Python reads \\ as a single \.
searchFolder = "C:\\Users\\rohrl\\OneDrive\\Python\\PictureCompare\\MixedPictures"