Geograph - photograph every grid square

You can download any image from Geograph and print it. If larger sizes are available it is best to select the largest for printing purposes. Do read the 'Find out How to reuse this image' page for details on the licensing, crediting and any constraints on reuse.

Use the Browser search and add "larger:1024" (without quotes) to the end of your keyword query (works with 641, 800, 1024, 1600, 3000) - example https://www.geograph.org.uk/browser/#!/q=bridge+larger%3A1024

... restricts the results to images that have resolution at least 1024 pixels on the longest dimension.

Use the 'Upload a larger size' link on the photo (or edit) page and submit the size you want (or select the preferred option on the page). This will overwrite the original selection although there may be a brief period when the originally selected size will be available due to caching and the wait before approval.
Be aware that if the very large version has been on the site for a while it may already have been downloaded from the site as a CC licensed image.
Note: if you did not want to license a larger size at all you will need to go through the same process but using an image marginally larger than 640 pixels wide (641 is enough) in order to remove the larger size options.

It is not possible to replace the image on the photo page but you can upload a larger size version of the same image with different/better processing and note the fact it exists in your description.
We are reluctant to remove images which have been on the site for a while (no telling where they have been linked from) so this is the preferred method of adding a better quality version.

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.