The photos you store in Windows' Pictures folder are typically the full-resolution images from your digital camera. Depending on the particular model, these can vary from a couple of megabytes for JPEGs to tens of MB if you shoot in RAW mode.
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Such images aren't ideal for emailing to friends and family, especially if you want to attach several to the same email.
For some reason, this never became a built-in feature in Windows - all you can do natively in Windows is to rotate a photo clockwise or anti-clockwise 90 degrees.
Once you've installed ImageResizer, you can select an image (or several images) in a folder. Then, right-click on one of them and choose Resize pictures from the menu that appears.
A window will open where you can choose the size. The original images won't be touched, instead new copies will be saved in the same folder. As a guide, the medium option is the best size for viewing on most laptops, while large is best for PCs.
That's it. Now you can email several pictures in each email, and they'll still look great on your recipient's screen.