Systems like linux (and probably mac, I don't use them) require filenames to have exactly the correct case. To them, Fwalk2.png.import and FWalk2.png.import are completely different (and incompatible) names. So you need to make sure the names of the files match the names to try to access them with.
Windows remembers the case of filenames, but ignores the case when opening a file. So this would run fine on windows, but fail on linux.