Yep, I make mine 2500 x 15xx or so depending....I generally work on them at higher resolution in Photoshop and save a jpg at the appropriate size. In checking I see my most recent one for Growing Up Stories was actually 3467 x 2600 (for whatever reason?) but The Poet of Utah Park was only 1000 x 750 ?? (maybe that's why it's not selling so well
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I also create a thumbnail to use as needed about 200 x 1xx or so...