File names should contain only letters, numbers, underscores, and dashes
A dash or underscore should be used instead of a space
No special characters (& ‘ ” ; : * ! # $, etc)
Lowercase letters are recommended for consistency, but uppercase or a mixture will still work fine
Source: http://www.lexiconn.com/blog/2011/02/best-practices-dos-donts-for-file-names/ an oldie but a goodie.
Image publishing guidelines, and having images be a part of search results in Google.com
From Google Webmaster Tools http://support.google.com/webmasters/bin/answer.py?hl=en&answer=114016