I purchased items to use as a backdrop for craft items I sell. Are they considered an advertising cost or supplies?

They would be deductible as marketing or advertising costs.  Even supplies would be okay if they aren't expected to last more than a year.

If you don't keep an inventory , like most craft sellers, then, if you sell them later, you could just include them in general revenue.  Even if you keep an inventory, you would still put them in general revenue but not allocate any cost of goods sold to them.