Geograph - photograph every grid square

There are several ways to identify empty grid squares depending on whether you prefer a visual map, a list, or a mobile-friendly view:

Using the Interactive Coverage Map:
The main coverage map does not have a dedicated "no photos" filter, but you can identify empty squares visually. Go to the map and enable the 'Coverage - Standard' layer. On this layer, squares with no photographs are shown in Green.

Tip: You can personalize this map to show your own contributions; in the personalized version, non-photographed squares will appear unhighlighted.

Link: https://www.geograph.org.uk/mapper/combined.php

Using the "Nearby Squares" Search (List View):
If you prefer a direct list of empty squares rather than a map, you can use the Squares tool. Simply enter a grid reference, and the tool will generate a list of nearby squares without images. You can also adjust the filters to find squares with "few" images or "no recent" images.

Link: https://www.geograph.org.uk/squares.php

On Mobile Devices:
If you are out in the field, you have two primary options:

Geograph Radar: This mobile app displays your current location and provides a count of images in nearby squares. Note that this tool does not use a map background (intended to minimize data usage).

Link: https://m.geograph.org.uk/radar/

Mobile Browser: The standard Coverage Map (linked above) is designed to work on small screens. Enable the Coverage layer to see the green empty squares while on the go.

For GPS Devices:
If you want to load these locations into a handheld GPS unit, you can generate a downloadable GPX file of squares matching your criteria.

Link: https://www.geograph.org.uk/gpx.php

Either:
1) Resize the image before you send
or
2) Limit the size of the image before you take it.

Most smartphones allow you to tweak the size of the image before you take the pic. You might find editing software for your smartphone to help you edit the image to get the size below 8MB.

Most GPS receivers allow you to change the datum and display format to OSGB36 and British National Grid, the convention used by the Ordnance Survey of Great Britain. If you're using a GPS built into a mobile phone, this may not be possible, and your coordinates will always be in WGS84 lat/lon format. If you also use your GPS phone to take your geographs, it should store the positions in the GeoExif header, which Geograph should interpret correctly as camera position (via the Geotagged Image option).

Otherwise, you can use the map part of the submission routine ('locate/find on map') and enter your lat/lon coordinates in the format the 'Search' function, in decimal degrees like 56.45657,-4.54675 . This will look up the grid reference for the location and insert it as _subject_ position into the submission dialogue. You may have to adjust this unless the subject was right in front of you.

Alternatively, there is a conversion tool here on Geograph Tools http://www.nearby.org.uk/coord.cgi?f=conv from which you can copy and paste. Or one built into geograph at https://www.geograph.org.uk/latlong.php

(Question asked by Roga and answered by barryhunter, oasthouse and DHL on the forum http://www.geograph.org.uk/discuss/index.php?&action=vthread&forum=18&topic=13483&dontcount=1&page=0#1
Later updated, as conversion tools have changed over time))