There's nothing to show definitely who sent this post card, and the place given in the postmark isn't clear. But I think it's Will. Here's why:
Will seems to have preferred sending postcards to sending letters.
There's an undated letter from Will and Margaret that mentions a lost cat. It was from around this time, but I still haven't hit on an exact date for it - but it's there.
Will loved cats.
Will occasionally drew.
The postcard was sent to Alma care of E.C. Evans in New York. A sibling would know that Alma was visiting Eva in New York, and a sibling would know that Eva's middle name before her marriage was Christine.
I can't see Fred or Laura sending this postcard. I know that Will could draw, but I don't have any indication that Fred could (granted, it wouldn't surprise me), and Laura definitely doesn't seem the type to do something whimsical like this.
I just wish I could figure out the location in that postmark.