Images

"Your PNG or JPEG bitmap image goes into its proper /res/drawable folder depending on its pixel dimensions. It can then be referenced by its file name only (excluding its extension) in the Java code, as well as in XML. For this important reason, be sure not to give a PNG file and a JPG file the same name, or you will have problems when it comes time to compile your code.
Also, contrary to normal file-naming conventions, image file names can contain only numbers, an underscore character, and lowercase letters, so make sure to remember this rule (one of the many anomalies of Android programming)."