JPG with minimal compression; IRCC suggests about 240 KB and never more than 4 MB.
The photo must be taken within the 6 months before you submit the application.
Altered photos are refused: no filters, retouching or background swaps.
How to do it with Requisito Listo
Open the «Citizenship application photo (online)» template in the app.
Take the photo with the on-screen guide (or pick one from your gallery).
The app centers your face, makes the background white and gets the exact size and file weight.
Check the checklist, then upload or save it.
At the end you'll see a checklist: size, file weight, background and format. If something doesn't comply, it tells you what to fix before the portal rejects it.