Is there any way I can replace the HELP screen on prboom?

In doom.wad or doom2.wad, look for the help screen image in Slade. In your wad, make an image with the same name, size, and format. That should do it.

That's what I did, and it did work well in zdoom, but if I launch the wad in prboom then it'll show a help screen which is unique for prboom :/

I'm afraid I don't know much about prboom. I'll have to leave this for somebody else to answer.

If Pablodraw can make it and replace the endoom lump it can.It works on all engines,uses classic as a base