Headshot Photo for Secondaries

I know schools require us to attach a picture of ourselves when submitting the secondaries. But I do not have any headshot photo of myself. Can anyone let me know the standard dimension/size? Does it has to be a particular color in the background? I have a yearbook photo of myself from a year ago that is 4x5 but grey in the background, will that do? Last, can I just take a selfie with my phone? Sorry If I sound dumb
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums