I am not sure what size's this revers to, X5 will make them smaller because of speed / loading time advantages. (think especially on mobiles and non wired connections)

In general 1000pix is about max, maybe 1200 but that's it for internet purposes.

But it could also be that sometimes It's better to make the pictures in the correct size before adding them in x5, espacially the aspect ratio. By changing that too many the pictures can get distorted. (it depends all on the picture, I sometimes cut them from 1 side if possible offcource...)

use Irfanview. Take care that pictures are not more then 1200 pixels for the longest site, more is not needed for a website gallery.

Soo make sure all pictures you use in the gallery have roughly the same aspect ratio and not too many pixels.