I am not sure what the process is to import a picture file into a texture file package and what settings are required to get a masked background (like pink or green or something), however I pretty sure you can achieve what you are asking in photoshop. (Assuming you have a copy...) You could open a new 256x256 pixel image and add in your other 55x59 picture as another layer. You could then center the new layer on the background by selecting it and using Layer->Align To Selection->Vertical Centers, and do the same for Horizontal Centers. That should get the picture perfectly centered. Hope this helps you some...

Oooooooh i totally forgot about its scripting feature... Hahaha i can definetly do this.. 

Nevermind, made a correct script. Something called "Canvas Size"