You can find all that information on USPS. You can put in the country (yours), then the shape of the envelope, and the weight.
http://ircalc.usps.gov/
Then it will tell you how much that will cost. You would want the price at the bottom of the list---the least expensive one.
You need to go to your post office and have your "return" envelope, and photo, weighed---you are only weighing what is being returned to you. If you are sending photos then you weigh those too. If you are not sending a photo, but are hoping they will send one, then take a photo you have to weigh it so you can get an idea of how much the total return weight is.
The weight is in ounces on USPS so if you need to convert to grams you can use an online converter.
Here's an online converter:
http://bilet.pp.ru/calculator/convert_o ... _grams.php
Once you know the cost then you can buy your stamps.