The optimal image size for images in Haiku Deck is 1124 x 768. Be sure to leave 50 pixels of space on left and right to allow for the cool scrolling "parallax" effect on iPad playback. In the example below, the gray box is the part of the image that you'll see on the screen. If your total image size is *less* than 1124x768, Haiku Deck will zoom in to fill the required dimensions.
Imported images are resized to 1124x768 and compressed very slightly for faster download.
One free web app for getting the job done quickly is Ribbet. Here are the steps for getting the job done quickly and easily:
1) Visit https://www.ribbet.com/
2) Click blank canvas
3) Type 1124x768, OK
4) upload a photo, choose the image you wish to upload
5) after the upload, grab the thumbnail of your uploaded photo from the bottom tray and drag it onto the canvas. Use handles to resize it so that it is horizontally centered with about 50px of 'buffer' on the left and right margin. This buffer will not be visible when you import into Haiku Deck.
6) Click save in the top/center navigation bar
7) Save image as png for highest quality and then return to Haiku Deck for importing