You can download your photos from Geograph in the same way anybody else can. In most browsers this involves right-clicking on the image and selecting 'Save image as' or similar, then choosing where to save it on your computer.
If you have licensed a larger size then pick the largest version on the download page.
Geograph does not retain unlicensed larger sizes so, no matter what size original you submitted, you will only be able to download the size you agreed to license.