@kadelex Is this right? Max 500 mb? Yours is 2500 mb? This doesn't sound right.. When I export my files as a full size PNG with no compression, they're usually around 5 mb and rarely more than 20 mb. Even my original PSD files, layered, are rarely more than 100 mb! How big is the size of your canvas? If you're scanning and the file size is this big, your settings are probably not optimal. You should be scanning at 300-600 dpi, any more is not helpful.
But back to your original question, I always suggest reducing the size first, see if you can get it under the file size requirement this way without making it too small, and then only if it's still too large play with the compression. Resizing makes the image smaller without affecting image quality, whereas compression keeps the image just as gigantic but reduces quality. For the purposes of the contest I would think you don't need more than 2000 px wide for your image. Try resizing it to that size and see if it goes under the requirement. For your image to be 2500 mb the size of it must be gigantic!