How can I wait unitl the image is done writing?

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I have a program, and it takes, and saves a photo at the start of operation. As is, I wait with a sleep, or delay, for an amount of time that Ive observed to be necessary to finish writing the image file completely, before trying to make a read. Surely, this cannot be the best way. unfortunately, I don't see anything that says when the write process is completed.

When writing image files, how can I be certain when the image file is fully saved?