While it would be slightly more costly, I can tell you that:
http://www.concrete5.org/marketplace/addons/ecommerce/features/...
would be perfect for what you need it for. I use it for my site and it has paypal integration right out of the box. It also has product lists so you can show thumbnails of all the pictures and once clicked on, it will take you to a product detail page. Both the list and detail page support lightboxes and callouts to enhance the customers ability to view the pictures. If you want to kind of see it in action you can check out my page athttp://www.yunotakemymoney.com but keep in mind I don't use ecommerce the fullest of its abilities because I don't quite need all of the functionality it offers. If you want to see a product detail page, click on one of the titles of a product as that is currently the only way to get to my product detail pages.

@kjsupe — The Ecommerce add-on could work, but I'm concerned it would be overkill. My client is not tech saavy so I need a simple solution. Too many options would be confusing.

@ScottC — Hard to say, but I think I need something more than just HTML buttons. The tcPhotoGallery allows a row of thumbnails on the page. You click on a thumbnail, and the larger photo shows above it. The thing I need is instead of a large photo showing, it would show a large photo, plus a PayPal button with size options.

Thanks for your suggestions!

No problem.

You might need to tweak a bit of css, but I think my solution would work.

Shoot me a PM.

-Scott

Colin, I hope you don't mind me jumping on your thread, but your requirements are very similar to my own.

Scott, I'd be interested to see your solution if you don't mind. My gallery/lightbox is highly customised so I'd love to see the optino of adding a button/link to it instead of replacing it outright.

kjsupe, I'm pleased to read your response. I've been considering eCommerce plugin for a while but never been sure if it'll handle my requirements. Would the idea be that I replace my gallery for that provided by the plugin, or would it allow me to integrate with my existing setup (my gallery opens a lighbox for each picture, not an individual page)?

I look forward to hearing from you, thanks for asking the question Colin!

Dan,

The add-on that ScottC provided to me is not publicly available and I do not know if he would sell you a copy. You can try PM'ing him to see if he will sell it to you. I have it installed on a live site and it works OK. Many of the options in it do not work correctly for me, but overall it is a good add-on. I did just experience a bad error with it that took down site content for a little over a week, but that's fixed now.

I may be replacing the add-on eventually. But for now, it works.

I don't know of any add-on that does what we need besides the one I'm using. If you do end up using the eCommerce add-on, please post back here with how it works for your gallery set up.

Good luck!

Colin