Three things that come to my mind:
1. Before calling glTexImage convert surface to RGBA format.
2. Disable depth test in SDL_GL_Blit procedure.
3. The (0, 0) texture coordinate corresponds to bottom left corner of the texture. So textTop and textBottom should be
Code:
textTop := (imgHeight -  SrcRect^.Y)/imgHeight;
textBottom := (imgHeight - SrcRect^.Y+SrcRect^.H)/imgHeight;