Why 800 × 600 will not die
It was a screen resolution before most of the people typing it were online. It survives because it got written into things: upload limits on forums and ticketing systems, attachment rules in old intranets, image guidance in documents nobody has revised, and application portals that specify a photograph in pixels rather than in kilobytes or centimetres. The number is not fashionable and it is still asked for every day.
It is also small. From a 12 megapixel camera you are keeping about one twenty-fifth of the pixels, which is the point at which how the resampling is done stops being an academic question. Drag the divider with the view set to 100% and you are looking at the real output pixels, not a scaled preview of them.
Fitting a modern photo into a 4:3 box
800 × 600 is 4:3, and that is the one piece of luck in this target: a phone photograph is usually 4:3 as well, so the chain holds and the height lands on 600 without anything being discarded. A camera shooting 3:2 is the awkward case. Fit gives you 800 × 533, which is the whole photograph and not the box. Cover gives you the box and takes a strip off the top and bottom.
If the form you are filling in cares about the exact numbers, use cover and check what the anchor kept. If it only cares that the picture is not more than 800 wide, use fit and let the height be what it is. Those are genuinely different requirements and forms are rarely clear about which one they mean.
The preset row also carries 200 × 230, which turns up in document-photo requirements far more often than its obscurity suggests. It behaves like any other pair of numbers here.
Questions we get asked
The form says 800 × 600 and also says under 100 KB.
Those are two different measurements and this page only answers the first one. Once the dimensions are right, the quality slider and the format choice are what move the kilobytes, and the number beside the download button is the real encoded size rather than an estimate — so you can drag until it fits. If the ceiling is hard, set it under File size: the page searches the quality for the best file under 100 KB at exactly 800 × 600 and prints the bytes it measured.
My photograph came out sideways.
It should not have. Orientation is settled before the resizer sees a pixel: the page decodes a tiny test image once to find out whether this browser rotates by itself, and only rotates by hand if it did not. If a picture still lands sideways here, the file itself is almost certainly wrong rather than the tag.
Can I go back to the size I had a minute ago?
Yes. Undo and redo cover the last twenty pairs of numbers, which is the only thing on this page worth undoing — the mode, the anchor and the format are one click each to put back.