The attachment limit you hit is set by whoever runs the mail server, at either end, and it varies. The practical consequence is the same everywhere: a few photographs straight from a phone can exceed it, and the message bounces after you have already written it.
Decide what the images are for
An image that will be looked at on screen needs far fewer pixels than one that will be printed. Sending a 4000-pixel photograph so that someone can see it in a mail window is the usual reason a message is too large.
If the recipient needs to print it, send it large but compress lightly. If they need to see it, resize it down first. Either way, resizing is more effective than compressing harder.
A workflow that keeps the files straight
- Copy the images into a folder, so the originals are never overwritten.
- Drop the copies into the compressor together.
- Set a maximum width in the resize panel - 1600 pixels is generous for on-screen viewing.
- Leave the quality at a moderate setting, and check one image at 100% zoom before committing.
- Download the results and attach those.
Each file keeps its own name, with the extension corrected to the format you chose, so nothing gets mixed up in the folder. The batch section of compressing images in bulk explains the naming.
If the message is still too large
Reduce the number of attachments before reducing quality further. Two images at a sensible size are better than six at a size nobody can read. If the set is genuinely large, a shared folder or a transfer service is the honest answer - email is not designed to carry a photo library.
Where the images are processed
Everything happens in the browser. The files are never uploaded to a server, which matters when the photographs are personal or confidential - as they usually are when they are being emailed rather than posted. The privacy policy says the same thing in more detail.