A single photo from a recent phone can be five megabytes or more. Understanding why makes it obvious what to change.
A phone photo can have far more pixels than a form needs. A PNG photo can also be larger than a JPG showing the same subject. Check dimensions and encoding first; metadata removal serves privacy rather than replacing image compression.
The starting maximum width is 1920 pixels. Choose Keep original size when dimensions must remain unchanged.
JPG and WebP use the quality control. PNG ignores it; compare actual sizes because results can be larger.
Start with the pixel count. A recent phone photo is around 12 million pixels, 4000 by 3000, while a full HD screen shows about 2 million. Most of that detail is never seen, so the quickest big saving is a smaller width, and the tool shows the new size straight away.
Then look at the format. A photograph saved as PNG can be many times larger than the same photo as a JPG or WebP, because PNG is not made for photographs. If yours is a PNG photo, converting it to one of the others is the biggest single saving.
Choose Keep original size instead of the default maximum width. JPG or WebP quality changes can reduce bytes but also affect visual detail.
A caption needs at least one line of text. JPG, PNG and WebP photos always open, and a very large photo may need a computer rather than a phone.
A full HD screen is 1920 by 1080, about 2 million pixels. A photo that is 1920 pixels wide looks sharp on nearly any screen.