Esri provides documentation Adding images to reports which provides a good overview and the steps needed to accomplish this in the Report Designer.
You must keep in mind that the setting of the image frame and location of image is key.
While you are designing the layout of your report, use a Picture control if you want to add an image to your report. Picture controls include an Image Source property that allows you to choose the source of your image from three options. The three image source options are 1) Choose a field, 2) Use a file path to a local, network, or web location, and 3) Use a static image.
I went about aligning the images horizontally and would set the image source for the report layout, as indicated below.