There are many different publicly available datasets out there, and most come with a paper describing how the dataset was acquired. Almost nobody takes a camera and starts taking thousands of pictures themselves. You may find some inspiration by looking at those papers and adapting their methods for finding images.
A very popular way is to download the images from Flickr: This is a photo platform where users share their photos and add comments or tags, describing the contents of the images.
Flickr also has an API to find and download images.
A couple of test queries show that there are thousands of photos available:
Query No of Matches
VW Passat 57,702
Ford Focus 187,344
Toyota Corolla 81,529
Mitsubishi Lancer 126,242
However, this is not a clean high-quality dataset: it includes old models, wrong tags, photos from the interiors, and so on. Still, it might be a good starting point to acquire huge numbers of images.
Maybe you can live with a couple of low-quality images, but I guess the better way is to clean the dataset up. There are a lot of different possible steps - some may not be needed in your case, and you might need other or additional steps:
- Remove non-car photos. You probably don't want photos from car interiors, or photos that don't show the car at all. You could e.g. classify all your images with an ImageNet classifier and discard all images that aren't recognized as "car".
- Use image retrieval algorithms (e.g. SIFT descriptors and matching) to build a graph containing all images and their similarities, as described in . Discard all images that have very little similarity to the rest of the images (or at least review those images).
- Manual labelling. This is the best way to ensure that you have a really high-quality dataset. Have someone go through all images and make sure that they satisfy all conditions you have and are labelled correctly.
This is very very expensive, but will definitely give you the best results. If you don't really need to - don't do this.
If you have to, you could rely on the Mechanical Turk or similar websites.
Flickr's API description says that:
The Flickr API is available for non-commercial use by outside developers. Commercial use is possible by prior arrangement.
Important note: all photos are property of their respective owners. All images on Flickr have specific license conditions, which you can also query through the API. A list of the available licenses on Flickr is available on their website. You have to make sure you don't infringe the copyrights of the respective owners. Especially if your work is commercial, this complicates things.
: Gordo, A., Almazan, J., Revaud, J., & Larlus, D. (2016). Deep Image Retrieval: Learning global representations for image search. arXiv: 1604.01325.