$select :
Use the $select query option to specify which fields to return for a given list, list item, or other SharePoint object represented by an entity set. You can use $select=* to return all available fields.
In General Case :
In general, if you do not specify the $select query option, the REST service returns all available fields by default. However, in a few cases, some SharePoint objects include properties that are very resource intensive to retrieve; to optimize REST service performance, these properties are not included in the default query, and must be explicitly requested.
(Source: https://msdn.microsoft.com/en-us/library/office/fp142385.aspx)
E.g.
var Ownurl = _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle(' Divisions’)/items?$select="ColumnName"&$filter=Title eq 'test1'";
In Your Code :
$.ajax({
url: siteurl + "/_api/web/lists/getbytitle('MediaAlbum')/items?$select="Page Image"&$filter=IsActive%20eq%201&$expand=ContentType&,
method: "GET",
headers: { "Accept": "application/json; odata=verbose" },
success: function (data) {
console.log(data);
},
error: function (error) {
alert("Error: "+ JSON.stringify(error));
}
});