I've come across an online archive called "Musee des Mondes Contemporains" which has a great photo album of WWI armor with hundreds(?) of images. (Sorry if it has been mentioned previously.) You can take a look for yourself here:
Note that while you can zoom in and out with the website's zoom features, I found it easier to just click on the download symbol at the top right. This opens up the image by itself in high resolution.
The website is in French but I found that some photos are labeled "tank" while others are labeled "char". Try various search terms related to your interest and see what you can find.
Please let us know if you see anything interesting! I've only looked at about 20 photos so far.