The common concept in our current society of an "angel" is that of a person with "feathery" wings. That of a "devil" is that of a person with bat wings with horns. We have the description of the merkava and other descriptions of people with six wings. We also have the description like a person with the legs fused into one pillar. In fact, it is actually a problem to use the term "angel" as too many of us automatically "see" the images from movies, television, or "religious" art in the museums. As a result, we should use the term Mal'achim, and avoid any attempt to translate it into English. I have spoken with teachers who have told me that they have had difficulties explaining some concepts when using the term "angel" that did not show up when using the term "mal'ach". THis is a practical matter rather than a matter of halacha.
Given the references to mal'achim (angels) many "appearances" are metaphorical only. The meforshim of the mal'achim that visited Avraham (and went to S'dom) explain how three mal'achim came to Avraham and two went to S'dom. Each malach is "created" for its specific task and only "exists" for the duration of that task. This is also the explanation of the reaction when Yaakov (and later Manoach father of Shimshon) asks the name of the mal'ach. The "name" of the mal'ach only exists in relation to its task. Once that task is complete, the name is gone.
Similarly, there were three tasks required in the visit to Avrohom. The mal'ach sent to predict the birth of Yitzchak finished his task and "left". The mal'ach sent to heal Avraham either had the rescue of Lot as part of that task or became "available" for a similar task afterward, or was just replaced by the mal'ach sent to rescue Lot. The third mal'ach, to destroy S'dom, was needed as part of the three, according to many meforshim, because the fate of S'dom was not completely decided until Avrohom showed the real chesed of his acceptance of the three "men".
A mal'ach can be a person, a natural event, or a supernatural being created and sent for a purpose. There are many examples.
A TSA official who delays someone so that he misses a flight he is not supposed to be on.
A woman and child taking the seat of a person, so that he can show chesed by letting them sit together.
A traffic jam to force a person to take a particular route.
A sudden rain or a wind to blow the clouds away.
And of course the mal'achim as we see in the case of Avraham, Yitzchak, and Yaakov as well as all the other times mentioned in Tanach.
Thus there is no specific "image". Additionally, one is not supposed to create images of the "residents" of heaven, but that is another question from the Ten Commandments.