You can use stock photos in a theme you have created as long as you are not selling the theme to other users. Themes are created by web developers for templates on wordpress, blogger, tumblr or godaddy for free or for purchase. If your theme is for re-sale you need to buy extended rights to the stock photo. If it is a free theme then you can use a royalty free stock photo or image on your theme.
Can I change the stock photo I purchased for my theme?
The stock photo can be used in its original form or in parts. If you wish to use a part or portion of the stock photo you do have the right to crop, blur, slice or change the color of the photo if this is needed for your design. The agreements do not state that the photo cannot be manipulated to suite your needs, the agreements just define under what conditions a stock photo can be used.
Consider an extended agreement
Royalty free photos come with non-exclusive agreements. This means that other designers can buy the stock photo from the stock agency. Your photo could be seen on other peoples creations if you have chosen a popular stock photo. To avoid choosing a popular photo and guarantee that you have bought a unique photo that will make your theme special, consider an extended licensing agreement option. These agreements are more expensive, but you will have a distinct design that is a true reflection of your ability and creativity.
If you buy an extended license from the beginning, you will have the option to sell your theme if someone wishes to buy if from you or if wordpress wants to put it in their library of templates.
Check Shutterstock for extended licensed stock photos and buy the right license from the beginning so that you can sell your template immediately if someone offers to buy it.