CSS for a custom field

I made a custom field for a content type, one that I may use on additional content types.

It is an image field that provides an "Article Image" it's machine name is field_art.

My question is: "Is there a small section of code I can add to my style.css that will make any image of that field type float left and apply the margins I desire?

I tried adding the following to CSS per some other posts I read and none worked :

.field-art .field-item img {
  float: left;
  margin: 0 10px 10px 5px;
}

.field-art  {
  float: left;
  margin: 0 10px 10px 5px;
}

.art img {
  float: left;
  margin: 0 10px 10px 5px;
}

Please help, I have googled this and found several pages but none have the same answer and any variation of what I have read has failed to get my images floating.