If you do not crop it, something else will
Upload a wide photo somewhere expecting a square and it gets cropped anyway. You just do not get a say in where.
Most platforms take the middle. Some run face detection and centre on that. Some crop differently on a phone than on a desktop, so the same image is two different pictures depending who is looking.
Cropping it yourself first is the only way to decide what survives. It takes ten seconds and it is the difference between a photo of your work and a photo of the middle third of your work.
Cropping is not how you make a file smaller
This one surprises people. Crop a photo to a quarter of its area and the file does not drop to a quarter of its size.
A JPEG spends its bytes on detail, not on area. Smooth sky compresses to almost nothing; a hedge, a brick wall or a crowd costs a lot. Crop the sky away and keep the hedge and you have removed most of the pixels and very few of the bytes.
If size is the actual problem, resize the dimensions or recompress it. Cropping is for deciding what the picture is of.
The crop itself costs no quality. Saving does.
Removing pixels does not damage the ones you keep, so a crop is lossless in itself.
What costs you is the save. Every time a JPEG is written it is re-encoded, and every encode adds a little more loss on top of what is already there. Do it enough times and you can watch a photo degrade.
So crop once and export once, rather than opening, adjusting, saving, reopening and saving again. If you are going to make several changes, make them all in one sitting.
Leave room if it has to work in more than one place
One image rarely has one destination. It goes on a website, into a post, onto a profile and into a document, and every one of those wants a different shape.
The practical answer is not to crop tight. Leave space around the subject so that whatever automatic crop happens next still has the important part in the middle of it. A tightly cropped photo has nothing left to give and loses somebody's head the first time a platform decides it wants a square.
Straightening always costs you the corners
Rotating an image to fix a sloping horizon leaves empty triangles where the corners used to be, so the fix is always rotate and then crop in.
That is unavoidable geometry rather than a limitation of any particular tool. The only way round it is to leave margin at the time you take the photo, which is worth remembering on anything you already know is a bit wonky.
Nothing leaves the browser
The image is opened locally, cropped locally and saved locally. There is no upload, no queue, no account, and no cap on file size beyond what your own device can handle.
Which for a photo of a job, a document or a person is the right default, and it is the reason this can offer no limits at all: there is no server to run out of.
Common questions
Does cropping make the file smaller?
Less than you would expect. Cropping removes pixels, so a photo cropped to a quarter of its area does drop in size, but the saving is nothing like proportional because a JPEG spends most of its bytes on detail and the busy parts often survive the crop. If the file size is what you are trying to fix, resizing or recompressing does far more.
Does cropping lose quality?
Not from the crop itself, because you are discarding pixels rather than changing the ones you keep. What does cost quality is saving as JPEG afterwards, which re-encodes the whole image and adds a generation of loss. Crop once and save once rather than opening and re-saving the same JPEG repeatedly.
What aspect ratio do I need?
It depends entirely on where it is going, and the platforms disagree with each other. A square works nearly everywhere. Anything vertical gets cropped on desktop, anything wide gets cropped on a phone, and every network reserves the right to change its mind. If one image has to work in several places, leave room around the subject so the automatic crops have somewhere to cut.
Why does my photo get cropped differently on different sites?
Because they crop it themselves. Upload a wide image to something expecting a square and it will take the middle, or the part its face detection likes, and you have no say. The way to control it is to crop it yourself to the ratio that platform wants before you upload, which is what this is for.
Can I straighten a wonky horizon?
Rotating and then cropping is the usual approach, and it always costs you some of the frame: turning a rectangle inside its own bounds leaves triangles of empty corner that have to be cut off. Leave a bit of margin when you take the photo if you know it needs straightening later.
Does the original get sent anywhere to be cropped?
No. The picture is cropped in your browser and nothing is sent anywhere, so the part you cropped out never existed anywhere but your own device. That matters more than it sounds, because cropping is usually how people remove a face, a document or an address from a photo before sending it. There is no size cap beyond what your machine can hold, and no queue.