Hi there, when it comes to preparing images, there isn't much to do. If you have a paid plan, your client will have access to the image optimisation capabilities of Pagelime, but even with the free plan, you can crop images. All your client has to do is go to the image manager (you might have to enable this feature for their particular site) and then upload their images. They might want to create some folders to categorise images, such as a folder for sedans and one for hatchbacks.
For actually placing images onto the site all you need to do is create the page that will have the images, and then you could insert some placeholder images that were the correct size and in the HTML you'd have something like this for each image:
<img src="images/gallery_image_1.jpg" id="gallery_image" class="cms-repeat cms-editable"/>
I didn't have an id originally, but it's required in this situation, I didn't even add any styles in the CSS.
The part that affects the the functionality is the "cms-repeat cms-editable" part. That allows the client to click on the image in Pagelime and change it to any other photo they've uploaded to the image manager. And even more powerfully, they can duplicate it like any other repeating section.
For your situation what might be best is to have a <div> that holds all the photos of a vehicle and it's description, and then apply a "cms-repeat" to that div, and set a "cms-editable" to each of the individual elements within the <div>, like the <img /> and the <p>. That way, the client can "repeat" an entire vehicle section at a time, and still alter everything within it. As well, the "repeat" feature allows the element it's applied to, to be entirely removed form a page with one click!
I hope that solves all of your issues.
Good luck!