Is it okey to have many keywords with the same color as the background, so that they are not visible and have the <google adsens start> and <google adsense end> tags around these keywords? is it violation of any Adsense policies?
You do not want to hide or cloak any words on your site. Just use the adsense tags around keywords already in your site do not add hidden ones.
The problem is I am having a photo blog travelosphere.blogspot.com So, it is not possible to have many keywords on my site. Also, I dont want to disturb my viewers by putting the keywords with the photo. Thats why I like to have this trick.. But I am not sure, whether this technique is valid with Adsense Policy.
this is against google adsense and google quality guides. if you use that, you may be banned from google search
NOOOO, moreover this is not even in your financial interest since your ads will be poorly targetted and won't get any clicks.
Yup.. I am facing the same problem. Though my photo blog(http://www.travelosphere.blogspot.com) get some visitors, they are generating any money from Adsense. What you suggest to improve the Adsene targeting in my site?
I guess the issue is with Click Through Rate (nobody clicks on your ads) ? That's quite normal you have almost everything wrong. A must read on how to improve CTR Once you have read if you still have questions, don't hesitate.
Start by reading my article here : http://adsense4money.com/AdSense/Improve-Click-Through-Rate.php then search the web