You don't need a shed! Photos are usually sent in Jpeg format. It is best to use a program such as Irfanview click here to reduce the size.
I've found it is not an exact science, as it depends on the recipients PC and connection speed. You may find it useful to email photos, after resizing, to yourself, and experiment with different settings.
If you send a photo without resizing, it will appear too large and require scrolling.