Hi, Jay.
It's always easier to combine two categories than to split a single category into two later. If you have one category for "Stock Photo Revenue" and another for "Stock Photo Costs", then you can easily subtract costs from revenues to find your gross profit. But if you put both purchases and sales into a single category, it's not easy to figure out how much you sold and how much they cost you.
It's important for you, as the manager, to know how much you sold, as well as how much profit you made. How else will you know if your business is growing or improving? Sales of $1,000 less $900 in costs this year would leave $100 if you used a single category. Sales of $2,000 less costs of $1,900 next year would also leave $100 - but that would obscure the important management insights that your sales doubled but you made only half as much gross profit as a percent of sales!
The IRS also wants to see your revenue and cost numbers, not just the net.
Depending on the way you do business, it might be important to recognize the distinction between "costs" and "expenses". If you found a good buy on stock photos and bought a stockpile of them, you might not sell them all before year-end. In that case, you might need to take inventory and deduct only the cost of those that were no longer available for sale next year. This might turn on the subtle question of whether you "sell stock photos" or "use stock photos in the business of selling your design services".
Do you have a CPA or other accountant with whom you can discuss this and other management questions? If this is just a hobby, it might not be important. But, since you are using H&B and you want to know your profit margin, I assume you are quite serious about this. Some competent advice might be very helpful. Better, more coherent advice than you can get, one topic at a time, in a newsgroup. You've just started thinking about these questions; accountants have been thinking about them for centuries and have figured out a lot of answers. There's no need for you to try to reinvent all these wheels. ;^}
RC