A tip with artwork for Arrivee.
Line art produced on the computer - such as a map - I find that if it's looking good on screen you can simply doublesize the image (ie double the width, and height to scale) and it will look good, though smaller, in Arrivee. 2250px will fill the width of a page (allowing for margins, 190mm) so you can work with a map size on screen of half that, which is comfortably within most monitors or even a browser window - on my monitor 1125px is about a foot across, a very comfortable working size.
So the map example I put upthread, for example, is 560px, it only took me a few minutes and without any further work this would upsize to 1120 and print as 95mm wide on an Arrivee page looking good and sharp. (Tim would trim that - it's slightly too big for 1 column of 2 - 530px /~/ 90mm would be better.)
(Photos obviously you just submit a big file (big in terms of pixel count) that Tim can downsize to fit as appropriate (the full A4 is approx 9Mpx, 3600x2500 allowing a bit for bleed). Jpegs are fine, I've had a lot of photos in Arrivee over the years and all my recent submissions (including cover shots) have been jpeg only.)