Can't tell a lot from the picture, but I'll gather that the thrush is in the crease of the frog. I've had new horses come with that problem. Easier to deal with if it's not deep, but I'll take it as a case of the thrush being down deep. You don't need to buy all the "horsy" products they push out there. People have been successfully treating and killing thursh for a lot longer than those products have been around.
Clean out the thrush.
Rinse it out with vinegar (you can cut the vinegar with water if needed, but no more than half). I usually have it in a spray bottle so I can spray it right into the area that needs it. I generally follow this by squirting in some tea tree oil and wiping out the area with gauze.
Get some gauze or cotton. Soak it with teatree oil. Pack it in so that all the area that has thrush is covered. If it's deep then I usually pack in all the cotton it will hold and make sure it's saturated with the tea tree oil.
I've seen it left in for a week before replacing, but i'm pretty anal about thrush and replace it after 3-5 days.
When you replace it rinse it with vinegar again before repacking with fresh teatree oil soaked cotton.
I've never had case that wasn't cleared within a couple of weeks max, but I keep rinsing daily even after the thrush has cleared until the crease reduces. Thrush loves those creases. If it shows up in pockets along the frog you can usually trim off the frog so that the pocket is completely exposed and then clean and rinse with vinegar daily. I tend to do overkill.
Vinegar works, because thrush apparently thrush has a problem with the acid. Some people like using apple vinegar and it works well (makes the feet taste better
If you don't have a choice about your horse being in an area that is likely to promote thrush then I'd suggest getting a spray bottle of vinegar and give the frog a good spraying every day when you clean each foot.