Hello,
I copied the logo string from ui.cpp of Repetier 0.92 (there the logo is possible) to the ui.cpp file of 0.91 version. You have to do the following steps:
1. Copy the string
u8g_DrawBitmapP(&u8g, 128 - LOGO_WIDTH, 0, ((LOGO_WIDTH + 8) / 8), LOGO_HEIGHT, logo); in your ui.cpp file of ver 0.91 just right before the string
for(uint8_t y=0; y<UI_ROWS; y++) displayCache[y][0] = 0;
2. Copy the string
#include "logo.h" in your ui.cpp file just right after the string
#include "u8glib_ex.h"
3. Copy the logo.h file of 0.92 versions to your 0.91 directory. You can take the attached file also.
4. Change the bitmap hex value for your own logo. I think you could also change logo width and logo height.
I have added a picture where you can see how i changed the values in my ui.cpp file
Alex