Different behavior in response if specified Accept: application / json

The Http method used is GET, and I call the same webservice method (with the same parameters).

But if I add an Accept header for the request with the / json application, the output is different. The reason is the Bitmap field in my object named User, which contains an avatar image.

If I do not use the Accept / json header, this is simplified output (XML):

<!-- language: lang-xml -->
<ArrayOfUser xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.datacontract.org/2004/07/XTraN4ForcesFSDomain.Domain">
<User>
    <Id>02ddf1e4-ad76-4778-8887-a186014939f8</Id>
    <Avatar xmlns:d3p1="http://schemas.datacontract.org/2004/07/System.Drawing" i:nil="true" />
    <IsActive>false</IsActive>
    <LastAccess>0001-01-01T00:00:00</LastAccess>
    <Username>quisquam</Username>
    <!-- Other properties -->
</User>
<User>
    <Id>17db833c-5008-44f0-a713-a186014c22a5</Id>
    <Avatar xmlns:d3p1="http://schemas.datacontract.org/2004/07/System.Drawing">
        <Data xmlns:d4p1="http://www.w3.org/2001/XMLSchema" i:type="d4p1:base64Binary" xmlns="">iVBORw0KGgoAAAANSUhEUgAAAIAAAACACAYAAADDPmHLAAAABGdBTUEAAK/INwWK6[...]BIJS/Wd6Pgu/mOoS/HADwfwFUI4VkJHOgAgAAAABJRU5ErkJggg==</Data>
    </Avatar>
    <IsActive>false</IsActive>
    <LastAccess>0001-01-01T00:00:00</LastAccess>
    <Username>labore</Username>
</User>
</ArrayOfUser>

Ok, that’s just great! Image (Base64) is. If I change my request for JSON, it will not receive any image, just the name of the class it represents:

<!-- language: lang-json -->
[
    {
        "Username": "quisquam",
         "LastAccess": "0001-01-01T00:00:00",
        "IsActive": false,
        "Avatar": null,
        "Id": "02ddf1e4-ad76-4778-8887-a186014939f8"
   },
   {
       "Username": "reiciendis",
       "LastAccess": "0001-01-01T00:00:00",
       "IsActive": false,
       "Avatar": "System.Drawing.Bitmap",
       "Id": "17db833c-5008-44f0-a713-a186014c22a5"
   },
]

Mthod web service -

<!-- language: lang-c# -->
public IQueryable<User> Get()
{
    // return stuff (no big deal here)
}

The code is the same, so why doesn't JSON return a base64 string, how should it be?