A problem with WAD characters in multiplayer mode

I have made 2 wads. To put the frames, i have "replaced" the sprites with the replace option. I can play with them with no errors in single player, but when I try to put them in multiplayer the game crashed (when I go to setup player and change "Sonic" to "my character") :(

It s a really horrible problem, help will be very appreciated! (If you know how to fix it, i will send you the wads in a PM)

Well, I think it might be that the picture sprite is too large. I know a wad where this has happened before.
Edit:What Darkbob said in case I was unclear.

Mabye theres a problem with the walking frames shown in character select?

the walking frames are OK, they work perfect in SP and MultiP, the problem is when i try to change from "Sonic" to "my char"

Anyway, i have played with them in a server using the "skin XXXXX" in the console

Not the running frames, the running frames that face YOU.