All booking engine images are one standard size 900 x 503.
The maximum file size is 1Mb
You can use any picture resizing tool (free online) to resize your images. Here is one example http://www.picresize.com
BKE image locations
1. Rate plan images - change on Step 1 of the rate plan
2. Room type images - click on edit for each room type and change here under both - 'Featured Image' and 'Image Gallery'
3. Grouped Specials / Packages - add/change image for each package by clicking on edit for each package