Welcome to using a UNIX-style operating system. Unlike Windows and DOS, filenames are case sensitive in Linux. .PNG and .png are not the same, and despite PNG being an acronym like most extensions, the convention is to always use lowercase.
This is a no-brainer - simply rename the files so that the extensions are lower case. You don't have to do it one by one, a simple Perl script or shell script could do it for you.
Perl / MySQL / HTML+CSS