Not sure if you ever found anything, but I had the same problem and found the following life saving link... it has italic, centered, right justified, left justified, and underline. The only thing that is missing is bold, which is pretty simple anyhow.
php-imagestringright-center-italic
From the site:
"They even work with fonts you load via ImageLoadFont, which is pretty awesome! You can even nest effects, to create for instance, right-aligned italic strings with a drop shadow!"
There’s an optional final parameter on each function, $ImageString, which contains a string representing the function to use to write the string. So for instance, if we want to write a right-aligned string in italics, we can use the following code:
ImageStringRight($image, 5, 25, 'Right + Italic', $col, 5, 'ImageStringItalic');