You can upload images of any dimensions, portrait or landscape, but the file size needs to be under 8 megabytes. We do resize them so their longest dimension is 640 pixels on the main photo page. Optionally, you can also release larger versions of various sizes for downloading and re-use.
http://www.geograph.org.uk/article/Larger-Uploads-Information
We only accept JPEG encoded images. If you have any image in another format it will need to be converted.
Ideally images shouldn't have a longest dimension of fewer than 480 pixels. While we might accept such images if they hold particular interest, we would really prefer a larger image.
We do record the EXIF headers from your original image, so it is advantageous to upload your original camera image or use image editing software that maintains the EXIF data if you want this information to be kept (but we don't currently make use of the data).