Phone cameras produce images much larger than most email readers need. Resizing a copy to a practical display size can make a large difference before compression.
What should you consider?
Keep the original and create an attachment copy. JPG is a practical default for photographs.
Practical workflow
Start with the destination requirement, make a copy rather than overwriting an important original, and compare the output at the size where it will actually be used. If the file is still too large, consider whether resizing, compression or a different format solves the real problem.
Related tool
Tip: The smallest file is not automatically the best file. Compatibility and visible quality matter too.