1. Although I suppose Quest Pro resizes JPEGs, etc, as required, is there an
preferred maximum original picture size (in pixels) to fill as much of the graphics window as possible?

2. Is it possible (via a script?) to centre pictures horizontally and/or vertically?

1. Don't know
2. No - that might change, by the looks of this

Actually Quest doesn't resize pictures automatically. It can resize pictures in a popup window (the old way of showing pictures), though it doesn't use resampling so it won't be "smooth".

So there's no preferred graphics size. I think the most common screen resolution these days is still 1024x768 or maybe 1280x1024, so bear that in mind when choosing a size for your pictures.

You should be able to centre pictures by using text formatting codes. If you print a message containing "|jc", subsequent text (and pictures) should be centred. To change back to left-aligned text, use "|jl". You can find shortcuts to these by pressing the "T" button in the Script Editor above the "Message" box. In QDK 4.1 these will be more user-friendly and you'll be able to press a toolbar button just like in a word processor.