An image that is already the right size (1024x768, say) will only be around one megabyte or less on disk and will not need resizing. It will, therefore, take less than a second to load and display.
Larger files can take exponentially longer times. If the image is a 6 Mbyte one (even if it is JPEG’ed down to one), reading it and expanding it to a bit map takes MORE than six times as long. Also, resizing to the display size will take even longer than the initial read. This can, in extreme cases, result in the image taking several seconds to display.
For this reason, it is important to ask all entrants to use the correct resolution. Where they do not, it is advisable to resize the images for them - providing your club rules and the entrant allow it.