I've entered a couple photo contests over the years. My advice? Enter the images you personally like and try not to judge which ones you should enter, by trying to see them through someone else's eyes.
With one contest I entered, I was allowed 10 photo entries. So I scoured my files and found 7-8 images I thought were worthy. Images I thought other people would like as well. Wanting to take full advantage of all 10 entries, but still lacking a couple to make up that 10, I found a couple shots that I thought were "ok" but probably wouldn't win. Guess which image won. You got it, one of my toss ins to make up 10 entries.
Simultaneously, I entered a second contest. Did not know at that time the results of the first contest I entered. Guess which photo won that second contest. You got it. The same damn photo that won contest number one. LOL!!!!!
ETA: That winning photo came in second place in both contests. Second place in the "artistic category" in one contest, and in the "people category" in the other contest.