Does crawler sees if we have picture as a background on the site and do you set background color when you use picture for background? some people told me to set a background color, because bot doesn't sees picture in the background and if is default background color white and the text is white -> google could see that as hidden text .. is this true ?
Yes it is true. You can check it by looking sites cache. Also you can try to look for keywords that written as hidden text.