Question

Usual image with the size equivalent to A4 or smaller when being processed, decreases license pages counter by 1. The exact formula may be checked in this article.
So, for the documents with a high resolution/DPI, it may take more than 1 unit. But sometimes it may happen that the image has low DPI and high resolution which provokes OCR technologies to count it as several pages. Let`s imagine we have a document with a resolution of 1920*1080 pixels and 96 DPI. It has (1920/96)*(1080/96)=225 inch² which will be counted as 2 A4 pages. With overwriting to 300 DPI we will have (1920/300)*(1080/300)=23.04  inch² which will be counted as a single A4 page. 

Answer

1. Create PrepareImageMode with defined DPI (below is C# sample):

       FREngine.PrepareImageMode pim = engineLoader.Engine.CreatePrepareImageMode();
               // Overwrite Resolution
               pim.OverwriteResolution = true;
               pim.XResolutionToOverwrite = 300;
               pim.YResolutionToOverwrite = 300;

2. Add the document with this PrepareImageMode. 

   document.AddImageFile(imagePath, pim, null);

3. After that, it may be processed: 

   document.Process(null);

Still. the above recommendation may not be sufficient if the document is of both high DPI and high resolution.