Our Foundry Health Support can change/update the background images for our customers ClinSpark login pages.
Although, there are no known size restrictions, we recommend using images in JPG format and 1920x1080 size range.
darker images are better (because of the white reset text not showing well).
Landscape format imagery feels intuitively better (4x3, 16x9 etc.). Never tried portrait or square though.
images that are small (file size in MB and/or pixels) look bad.
images that are very big (MB) may slow page load times.
Images are ‘locked’ at top left, rather than centred in the display, so if you need/want to display logos or important messages in the artwork then it is better to have these in the top left quadrant of the image.