This is very unpredictable, especialy when using pivot tables. Your source table will shrink or expand and you usually have no controll over it.
My suggestion is to play with size of the page of PixelPerfect sheet as opposed to trying to resize image. Resizing image will not solve issue.
Make sure you allow enough space to export image in its original resolution an chart size (keep in mind that QlikVIew Pivot table chart) may have set size by disabling "Size to Data" option on Layout Properties and setting custom size in Caption Propierties.
Once you go past this step drag and drop your image on your PixelPerfect sheet and see how much bigger you need to make your sheet in order to fit your image.(maybe reduce margins if required)
On the other hand - why pictures? Cant you build object (pixel perfect table) to do what you want?
I would to suggest you to set the Sizing option of the image you imported in PixelPerfect report.
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