I had a look at this since I was not sure what the options were myself.
All tests done with a mug shot. No weirdness occurred. All images look "OK".
1.
Using Paint - Pictures get resized - pixel count reduced.
This may or may not be acceptable with respect to passport requirements.
ugly-mug.jpeg 161k
Windows Explorer
Right click "ugly-mug"
Open with "Paint"
Resize to 90% - funny little two squares icon near "Select". Maintain aspect ratio.
Save As "ugly-mug-90"
Close paint
ugly-mug-90.jpeg 141k
Windows Explorer
Right click "ugly-mug" -- start with original each time seems a good idea.
Open with "Paint"
Resize to 80%
Save As "ugly-mug-80"
Close paint
ugly-mug-80.jpeg 116k
ugly-mug-70.jpeg 91k
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2.
Office Picture Manager - This tool is designed for multiple picture batch processing but works fine on a single picture too. It is even included in my Office 2010 Starter which came free with the computer.
It offers a reduce size option but I have not tried that here. I "exported" the picture and chose a different jpeg compression level.
This has the property (advantage?) of retaining the same pixel count.
I moved the file into a folder that Picture Manager could see. It only sees "picture folders" as defined by Microsoft and by you.
Select picture
Click "File-->Export" Export toolbar appears on RHS (in my case)
Click "Jpeg Options ..."
Click "Custom compression setting" - choose a compression %.
Click "Return to export"
I changed the name of the file to preserve the original
Click OK - This actions the Export of the file.
Original file 161k
90% jpeg compression 121k
80% 70.5k
50% 34.4k
Let's see if % is an area based rule.
Original 161k
90% -> 81% of area -> 128k
80% -> 64% of area -> 103k
50% -> 25% of area -> 40k
Not really but it seems quite close. Might be worth a try as a first cut.
Good luck.