I read some where that in on-page optimization avoid graphics links....but i didn't get that actually what are graphic links ? & why we avoid this?
That does not matter, if graphics are being used, do not forget to give the "alt text" for the images which are interpret them in a site.
yes, this is good idea, you must give images alt tag. many of my such images are coming on google images first rank.
Yes, it's good idea to SEO on Google Image, don't forget some important thing like : filename, image host, title, alt, description