First you have to save the photo. Depending on the email program, this means right click on the image and chose "save as" or using the menu option "copy attachment". Now you have the image on your computer, for example in my documents and ca resize it as you do with any regular image file (see the linked article for an answer to that).
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If you copy the photo into MS-Word for example, you can show it smaller. This won't reduce the file size of the photo, and no image quality will be lost.You can also use an image viewing program, such as IrfanView, for this purpose.
If you actually want to make the file size smaller, you will automatically lose image quality as well.
IrfanView, which is freeware, can also help you with this.
Open the file, reduce the image size, save it again - to the same location, or to a different location, or with a different filename, if you want to keep the original, high-quality, image.
IrfanView also has options for bulk conversion: convert all images in a specified folder, for example, reducing their hight and width to 50% of the original.