When you rename an image file on the res folder show following error message:
Invalid file name: must contain only [a-z0-9_.]
ie, file name must start with a lowercase letter, then it may contain lowercase/uppercase letters or numbers or underscores or dots.
Select file name on the Package Explorer then
click the right mouse button -> Refactor -> Rename
or
press Alt+Shift+R
and insert valid file name.
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