If you click on a thumbnail from the catalogue page, you should get the small size of the image (not a thumbnail version), and then usually
a click on that image brings up the largest size available.
Right click the image and choose 'Save image as...' and you should have downloaded the best image the member provided.
I cannot understand at all how you are downloading thumbnails - it is not an error any of us have come across before.
If it happens again, please detail for us exactly what you are doing and what happens at each stage and we'll see if we can help.
If you can provide a link to the disrupted page, that should help too.
If you, for example, to this page
showing the gallery of bears, they should all appear as t/nails and, as Carol says, you can alter your viewing to up to 90 images per page.
NB - Now, if you have increased the number per page to view, there was a period where some of the images appeared in a larger size and upset the page! At present, all is well and I have tested each option of that Gallery.
Even so, still clicking on the "larger" "t'nail" should have taken you to the correct image, and thus to a larger size.