The issue you are having would suggest the images are from a mobile phone and were taken in different orientations, ie some are vertical (portrait) and others horizontal (landscape), or if from 2 different devices, one of is a mobile phone.
In either case this is a common occurrence with mobile phone images where the orientation flag in is either not set or is not read by software.
Options are to rotate the images in an image editor and save them with a new name or rotate them in Sideshow Maker if possible.
If none of the above applies what are the source devices that the images were taken with?