I bought this postcard at an estate sale a couple years back and just came across it again.
This may not be a Halloween postcard, but it has a creepy skeleton jockey racing on a skeletal horse, so good enough for me.
I'm not sure what the expression refers to, probably one of those sayings whose meaning has been lost to time. It might refer to the expression "Working on a Dead Horse" which means work that has been paid for before it's performed. Likewise, I couldn't find any information on the artist, "Peyton". There are a few other examples of his artwork on the internet, most are similar to this. I believe that says "05" next to his name which may refer to the year 1905. The 1-cent postage on the back would support that.