Do you resize within the art program you use?
When I export my images, I always resize the photo in the art program I am using to help preserve some quality. I've found that online photo resizers tend to loose a LOT of sharpness in the image.
What size canvas do you work on for your art?
I do 1000x1000 for horse avatars,
900x900 for stable avatars,
and 2007x450 for stable banners.
Also, I recommend sharpening the horse before smudging and the slightly sharpening again right before you export. Sometimes it is very easy to smudge the horse stock a bit too much and loose lots of the depth.
Also another thing to be aware of, usually if the horse stock is very small on the banner, it tends to be quite a bit blurry. That's why many artists make the horse fill up as much of the banner as possible.
If you want the horse to be small, be sure to blur the background and sharpen the horse. This will help the horse stand out and not look blurry compared to the sharp background.