Some opinions....
I hope that my critiques will be taken as constructive criticism... I am NOT slamming your work. In fact many of the things that people tend to leave out you covered and I like the general setup and "flow" of the site. I have developed my opinions from the several sites I have made.
Here is my input for improvement for what it is worth:
1) The red coloring just does not do it for me. You are selling a warm rich earth-based product, not shampoo.

I would think that a wood grain or at least tan-brown might give you more of the "feel" you want. Wood especially decking is 'warm' and 'homey' I'd consider this in selection of color for the site.
2) Slightly related the red print on grey background can boggle some people's eyes and make it hard to see, much like bright royal blue writing on black backgrounds. To some it looks great others have a hard time with it.
3) You might want to consider a mailing address on your contact page. Also I'd consider the line about booking a consultation. Sounds too pushy to me. Rather you might want to invite questions and offer help. Sure it means the same thing, but it has a nicer edge to it.
4) Most importantly, I would consider getting a TOP NOTCH or better yet a professionally done photo for the front page. Again, you want it rich and looking particularly lovely. The (please forgive me) snapshot-like photos are fairly okay for a photos page, but you need someting that looks like a shot out of fine woodworking magazine for the front page. It should have depth, warmth, be easy on the shadows, be crisp and clear, be colorful, and be well staged. I would lose anything that has eye-pulling background that pulls your eyes away from the deck. You want your visitor's focus to be so totally on YOUR work that they don't see anything else. The cool house in the background and the covered BBQ in the current photo are examples of what I'd lose. Particularly for a front page photo.
5) As for your photo size worries, you can make the initial photos small and then give the user the choice of loading big, bigger, and stupidly-big size photos.
Hit this page and scroll down toward the bottom for an example of what I am talking about. You are VERY astute in keeping the page loads quick, but for those who are really interested there is nothing like an oversize photo (if it is a good one of a great product)
like this one to get the customer on the phone. Do give visitors the option of more eye candy link to a larger photo from small ones.
6) Finally, you want your pages to be STICKY. That means you want the visitor to stay on the site as long as you possibly can. This will mean that they go to many pages on the site, each time seeing your logo and being tacitly invited to click to your sales area. How to do this? One way is a FAQ (frequently asked questions) page or better yet set of pages. This is a good start. Better still is a detailed guide to selecting product, choosing a contractor, different materials and their relative value to a project, and whatever else you feel free to share with the competition and the world. Remember YOU know your product, the public does not. Giving them a chance to understand how a deck is made, what to look for in materials and workmanship etc are all important. They have time to kill on the web, let them kill it and learn it from your site. Thats your first favor to them, the hope is they will see it as such and CALL. It also keeps them on your site longer.
Click here for my more general ideas about website design if you are interested in the mindset.
I hope this helps a little.
Charles