Most things that ask for a smaller picture are really asking for fewer pixels. A 12-megapixel phone photograph is around 4,000 pixels across; a full-width website banner rarely needs more than 1,600, and a forum avatar needs 200. Choosing a width is the single most effective thing you can do to a file — it beats quality settings and format changes combined, because it removes information rather than compressing it harder.
One, or up to 20 to resize to the same width together.
The height follows the shape of the picture, so nothing is stretched. The estimate updates as you drag.
One file, or the whole set as a zip.
Step by step, with screenshots: How to resize an image
It removes detail rather than degrading what is left. Made smaller, a picture loses information it can never get back — but at the size it will actually be looked at, nobody can tell. The mistake is making something LARGER: those pixels have to be invented, and invented pixels always look soft.
Work backwards from where it is going. A full-width banner on a website: 1,600 to 2,000. An article illustration: 800 to 1,200. An email attachment: 1,600 is generous. An avatar: 200 to 400. Doubling any of those is reasonable if the picture must stay sharp on a high-resolution screen.
Very large pictures are capped at 4,096 pixels on the long edge and around 12.5 megapixels overall. That is not an arbitrary number: above roughly 16.7 megapixels iPhones and iPads abandon the drawing surface and hand back a blank image rather than an error, so the cap sits comfortably underneath it.
Not directly — the width is the control, and the height follows from the shape so the picture is never stretched. For a specific height, work out the width that gives it, or crop to the shape you need first.
Free Online Image Optimizer