This one was really hard! All of the colors seem to melt into each other, making it really hard to pick out any differences. We hope you got there in the end and didn't get too frustrated. We'll start with the easiest answer first. Did you notice the different saddle colors? In the first image it's brown and in the second one it is blue on top.
Now, check out the red rings underneath the horses. There's an extra little one that snuck in on the second picture that wasn't there in the first image. Last but not least, glance over to the pillar on the right. The blue decorations at the top are where you'll spot the final difference. In the first image they run all the way across and in the second, the last one is missing.