Create a graphics file that is 64 pixels by 64 pixels. When you export a project, it creates the default loading-logo file. That is the default loading logo.
Now, move your own 64x64 pixel file into the compiled folder. Make sure you copy and paste, so you still have it, otherwise your only copy will be in the compiled folder (where Construct 2 exported the project files to.)
delete the default file named loading-logo from the compiled folder. Now, find your 64x64 pixel custom made file in the Construct 2 exported files compiled folder. rename that exactly this:
Now, when you run the game, you should see your custom loading graphic instead of the default one.