If you own a Mcufriend style TFT, I have put a drawBitmap_kbv example in the latest version. This might help you.
If you only have a 128x64 OLED, the principles are the same.
- go to an online tool e.g. http://skaarhoj.com/FreeStuff/GraphicDisplayImageConverter.php
- browse for a PNG or JPG of your logo on your PC
- adjust pixel dimensions if necessary
- copy/paste the result to your sketch
Personally, I put data like this in a separate H file in a sketch tab. Then include the local H file.
There are several different PC programs that can create the bitmap array for you. And several online programs. Some are easier to use than others.
If you are having difficulties, post a link or attach a JPG of the logo that you want to use.
David.