Go to Aqua Mail's Settings > Composing and Sending ? Shrink large images when sending > Bring up the above screen at least once to enable the feature, and optionally, select a default “target size” which will be applied immediately when you attach an image. The “large” target size is probably what you’ll want: large enough for computer monitors, and yet producing reasonable file sizes.
On the compose screen, you will see original and estimated shrunk file sizes, for example: “Foo.jpg 5.2 MB ( ~ 1.4 MB )”, where 5.2 MB is the original file size, and ~ 1.4 MB is the estimated shrunken file size. You can use Menu -> Shrink images on the compose screen to adjust the already attached images.

Note : This feature works on JPEG format images. Availability varies with phone hardware and some older devices may not be able to utilize it.