I f you see a red X in the gallery it means your picture was not accepted. This happens if it is too big.
When we create the gallery in about 2000 we did not want a lot of huge picture files taking up our server space. Back then a 1 megapixel camera was common - now they are 10 or even 15-mehapixels. That is huge. One gallery could be almost 1 Gigabyte.
So, that is why we only allow smaller pictures.