Modern phone photos are large, and a handful of them will exceed most attachment limits. Shrinking them first is quicker than any workaround.
If attaching several photos, compare their combined saved sizes with the mail system's limit. A smaller screen-viewing copy can be useful, but keep larger originals if the recipient plans to print them.
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.
The limit applies to the whole message, not to each photo. Five photos at 3 MB each is already 15 MB before email makes everything about a third larger. The tool shows the total size of what you have added, so you can see whether the lot will fit.
Divide the limit by the number of photos, leave room for the extra a third, and aim a little under. A width of 1600 pixels at the default quality typically comes out well below 1 MB per photo and still looks sharp on a screen.
Choose Keep original size instead of the default maximum width. JPG or WebP quality changes can reduce bytes but also affect visual detail.
JPG, PNG and WebP always open. A phone may run out of memory with a big batch of large photos, so add fewer pictures at a time or use a computer.
Take the email limit, divide it by the number of photos, then reduce that by a quarter or so to allow for how email encodes attachments.