Your best option is to just use comps for any images that you may use for a client's website. They will have a watermark on them until you purchase the full rights, but that is all the client should need to get a feel for the design. I never purchase stock photography up front, only once the client is absolutely sure they want that photo. Even if they say they like it, I may wait until near the end of the project to actually purchase and replace the comp image with the final.
This will also allow you to bill the client for the stock photography before the final launch (as I usually break up a project billing cycle into separate milestones). The selection of the photography is around the mid-stage of a project so those would be billed before launch anyway.
Finally, even if you will be billing the client for the photos in the final payment, I would assume that you have a solid enough contract that you shouldn't have to worry about non-payment? Now I'm not saying that's a guarantee that you wouldn't have problems, but I'm saying that it should be solid enough that it's not in the forefront of your thinking. A solid contract and good communication with a client should keep any thoughts that they're going to skip out on a project and leave you without payment ... for the project or any stock photography.