What do you mean by ’image’ field? Like a file field? Or one that hooks into the MODx.Browser?
If the former, just add a ’inputType: "file"’ to your config for the field.
If the latter:
{
xtype: 'modx-combo-browser'
,name: 'fieldName'
,value: 'myDefaultValue'
,hideFiles: true
}