Given that Richard Connell does not provide an exact picture of what Ship Trap Island looks like in his text, one can only create one.
Connell only provides small details as to what the island looks like. Given that no one has ever made it off the island, there has been no detailed description given. There are only rumors about the island.
"OFF THERE to the right--somewhere--is a large island," said Whitney." It's rather a mystery--"
"What island is it?" Rainsford asked.
"The old charts call it `Ship-Trap Island,"' Whitney replied." A suggestive name, isn't it? Sailors have a curious dread of the place. I don't know why. Some superstition--"
"Can't see it," remarked Rainsford, trying to peer through the dank tropical night that was palpable as it pressed its thick warm blackness in upon the yacht.
This excerpt only sates the fears associated with the island. Nothing is truly known about the island itself. Even Rainsford, with his keen hunter's eyesight, cannot describe the island.
Later in the text, readers are given very few deatils about the island. From Zaroff's own description, readers are able to learn a little.
The island is perfect for my purposes--there are jungles with a maze of traits in them, hills, swamps--"
One other small detail given to readers is about the swamps in the southeast corner of the island. Again, Zaroff provides this information.
I suggest, too, that you avoid the big swamp in the southeast corner of the island. We call it Death Swamp. There's quicksand there.
Outside of the two specific descriptions of the island, only small hints to what the island looks like are given. The narrator simply describes the island as a "trackless wilderness" with a "thickly wooded ridge."