Here is one of them:"Clipping Path Outlines on photographs of objects are a handy feature that I haven't yet experienced on Dreamstime."
Although I've seen images with clipping paths this customer probably didn't probably download one of them. It is good to keep in mind when post-processing your photo that there is enough interest from buyers for this so try to provide a clipping path for the main subject in your image. If you work with layers or isolated objects that should be easy to do.
How to create one in Photoshop:
1. Separate your object in a different layer. Make sure its margins are well preserved and that you are extremely careful when isolating the object.
2. Select it by using the magic wand tool
3. Right click the selection and choose "Save selection". Assign it a name such as "name of the object".
4. Continue with whatever other postprocessing you want to make or save the file (always go for the highest quality, don't forget that).
5. When uploading it you will want the customer to know about a clipping path. you can include full details within the description area, enter "clipping path" within the keywords and maybe within the title: i.e. Object name (with clipping path)