Most photo problems are easier to prevent than to repair. The capture conditions matter long before the crop is opened in an editor.
Keep the units straight
Millimetres and inches describe physical output; pixels describe digital dimensions; DPI connects pixels to physical size during printing. If an authority already states the required pixel dimensions, use them directly. If you are building a print file from millimetres, the basic relationship is pixels = millimetres × DPI ÷ 25.4.
Where calculations stop being enough
A mathematically correct file can still fail because the face is the wrong size inside the frame, the background is unsuitable, the compression introduces visible artefacts, or the application expects a different file type. Finish every conversion with a visual and application-specific check.
For a time-sensitive application
Download or save the final file before closing the session and note the destination next to its filename. That small bit of housekeeping matters when several applications are being prepared in the same week.
The practical risk
Document photos are easy to underestimate because the final file is small. In practice, the receiving system can enforce precise dimensions or upload limits that a normal portrait does not meet automatically.
The part worth saving
Treat the prepared file as a deliverable with a destination. A clear filename, current requirement and preserved original make the workflow much easier to audit or repeat.
How to Convert 2×2 Inches to Pixels
Use an existing photograph or capture a new one, then prepare the image around the requirement instead of around a generic preset.