You need to get some understanding of PHP syntax. Specifically how function parameters work. In PHP (and most procedural languages) function parameters are positional. That means that, in the case of `getCustomImageURL()`, the first parameter is the size value, the second is the width, the third is the height, etc.
So, the call you want is `getCustomImageURL(NULL, 675, 550)`. You could also do a search on `getCustomImageURL` and see how other themes are using it.