I saw many people put other companies logos (images) on their website, some use just text, is it legal to put a Google logo image on your website ?
Two words: ask permission. Other people's behavior doesn't always tell you what's allowed and what's not.
You could without asking for permission the main thing is that you dont use the logo for your own use. Google logo should be for google purposes. But still to be on the safe side ask for the permission first.