Finally, I've managed to get a file less than 250KB, but I worked the other way round: I converted the smallest (less colourful) files first, so the quality loss isn't noticed. And, at least in my computer, there is a not-negligible quality loss (see the attachment), and I didn't want the main windows with those artifacts, so I converted cbuttons, text, shuffrep... and I left out the unused cursors (wsmin, wsclose, wswinbut, volbar) and numbers (because I had nums_ex).
I've submitted it to 1001 and I'll tell you when it's published (do you know how long it takes?). There are some little updates from the version at deviantart: text.bmp, kb/s & kHz, background mb colour.
Thank you for your help