"_1024" and "_512" represent the resolution of the image. It's better to choose images with bigger resolution - "1024".
Select images to download by right-click on the image name and selecting "Save link as.." or "save image as.."
Make sure you can open images from your device.
Click the green "Browse" button at the top of this webpage to start measurements.
You're welcome to use another resourses like ASSA at ISWA. The image might not be in 1:1 ratio in the preview, however it should be proportional when you download it as a file.