Unless the majority feels otherwise, I suggest that only captions made on pictures that were in the photo contest should be eligible to win the caption contest. I don't see any problem with people submitting additional pictures/captions, but I suggest that they shouldn't be eligible to win our grand prizes .
In the past, I have entered, and won the caption contest, with photos that were NOT part of the regular photo contest. Twice, I think.

My entry this time was not in the regular photo contest, and was not taken by myself either.

I thought the fun of this contest was to see what funny captions people could come up with, and I don't like limiting the source of photos at all.

However, I do like the photos to be pictures of Thamnophis species.

Just my $.02

KIT