What is the picture field in the node object? print_r($node) outputs:

stdClass Object
(
    [nid] =&gt; 4
    [type] =&gt; grouptype
    [language] =&gt; fa
    [uid] =&gt; 1
    ....
    [title] =&gt; sample group
    ....
    [name] =&gt; roshd
    [picture] =&gt; 
    ....
)


What is picture and how can I upload a picture into this field?
This is the profile picture of the author of that node. You can upload that in your edit profile form and you can display it using theme('user_picture', $node);.

If you want to be able to attach images to nodes, have a look at CCK and ImageField..