{
 "images1" :
    {
      "size" : "29x29",
      "idiom" : "iphone",
      "filename" : "Icon-Small@2x.png",
      "scale" : "2x"
    },
 "images2" :
    {
      "size" : "29x30",
      "idiom" : "iphone2",
      "filename" : "Icon-Small@2x.png2",
      "scale" : "22x"
    }
}

$ jq '.["images1"]' file.json
{
  "size": "29x29",
  "idiom": "iphone",
  "filename": "Icon-Small@2x.png",
  "scale": "2x"
}

jq --arg var "images1" '.[$var]' file.json

jq ".$1 | { keys: keys_unsorted, values: [.[]] }"

$ echo '{
 "images1" :
    {
      "size" : "29x29",
      "idiom" : "iphone",
      "filename" : "Icon-Small@2x.png",
      "scale" : "2x"
    },
 "images2" :
    {
      "size" : "29x30",
      "idiom" : "iphone2",
      "filename" : "Icon-Small@2x.png2",
      "scale" : "22x"
    }
}' | jq ".images1 | { keys: keys_unsorted, values: [.[]] }"
{
  "keys": [
    "size",
    "idiom",
    "filename",
    "scale"
  ],
  "values": [
    "29x29",
    "iphone",
    "Icon-Small@2x.png",
    "2x"
  ]
}