VK Web API (simple)

I would like to call the method "photos.get" and retrieve photos from a VK community. The problem is that I am unable to view the oid i.e owner_id( one of the parameters required for the request) when I visit the URL. For example, when I browse a certain community, I end up getting a URL like "http://vk.com/picsa". Instead of this picsa, I require a number that can be passed in as an argument. How do I get the owner_id instead from the URL?

You can use groups.getById API method. It does not require any authentication:

https://api.vk.com/method/groups.getById?group_ids=picsa

You will get the default group entity in JSON format:

{
    "response": [
        {
            "gid": 37460919, 
            "name": "AMERICA", 
            "screen_name": "picsa", 
            "is_closed": 0, 
            "type": "page", 
            // photos of different size go here
        }
    ]
}

Now, you can extract the community's ID (gid) and will be able to use it in other API requests. You can get information about several communities at the same time - you need to enumarate them with commas (/method/groups.getById?group_ids=picsa,othergroup,somename).

When you work with photos.get method, keep in mind, that VK uses negative ID values for communities and pages.

For example,
/method/photos.get?owner_id=123456789 means "photos of user 123456789" and
/method/photos.get?owner_id=-123456 means "photos of community 123456".