Hello there,
You need to paste your passport size photograph on the admit card and on the self declaration form. Remember to carry 2-3 passport size photographs with you on the day of the examination since you need to paste it on the attendance sheet. The postcard size photograph which you have should be pasted on the proforma which comes along the NEET admit card. Remember to carry 2-3 postcard size photographs as well. You can visit the article link given below for more information about the same:
https://medicine.careers360.com/articles/neet-2020-photo-size-format-documents/amp
Hope this answer helps you.
Good luck!!