What steps does an image editor go through to edit a JPEG?

Question

Say for example I want to a draw a red line across a JPEG image, what steps does an image editor go through to handle this?

Is it like the following?

1. Decompress JPEG into other file format (Ex. BMP)
2. Make modifications to image in this new file format

Answer 1

To modify the image you need:

1. decompress the jpeg file and store the raw image in the memory
2. modify the image
3. compress and store as a file in jpeg format