When making fine-art prints, more resolution is better, but on the web, it’s more complicated. You want to strike a balance between image dimensions, typical screen resolution and file size.
Ideally, people will want to scan your photos in a gallery of thumbnails and view selected images larger. If you use online gallery services like Flickr, the thumbnail and larger images are predetermined, and the various necessary image sizes are generated automatically for you when you upload the file. With Flickr, the largest of these is 1024 x 1024 pixels (unless you have a pro account, which has no file size limit).
Resizing your images to fit within 1024 x 1024 is a good target size for most monitor resolutions. It will be full screen for some viewers and slightly smaller for those with very high-resolution monitors, but still big enough to enjoy the photo. These pixel dimensions also will result in a file size small enough to download quickly. A file of this size will be ready for upload with most online services.
If you’re building your own web gallery, you should make some thumbnails, too. I prefer thumbnails to fit within 240 x 240, as this is big enough to get a decent look at what’s in the photo. Thumbnails much smaller than this make it hard to see useful details, especially with subjects like group portraits, when there’s a lot happening in the frame.
1. Make a folder on your desktop to hold the web-ready images. If you’re building your own gallery and generating your own thumbnails, make a separate folder for those.
2. Open your image file in your photo editor. If you haven’t made your typical postprocessing adjustments yet, make them, but don’t sharpen yet (sharpening usually should be your last step when preparing an image for output). Save the file in a lossless format like TIFF or Photoshop’s PSD.
3. Resize the image to fit within 1024 x 1024 pixels. For horizontal images, set the width to 1024 and let the height be set automatically. Be sure to constrain proportions to the original. If you’re using Photoshop, in the Resample Image drop-down, choose Bicubic Sharper.
4. Now apply some sharpening to the image. Use Unsharp Mask, if available. If you can toggle a preview of the effect on and off with your software, do this to check that you don’t oversharpen, which creates too harsh edges. Some photographers have formulas for sharpening, but I like to judge sharpening by eye for each image use.
5. Save a copy of the file in JPEG format in the folder you created on your desktop. I recommend a naming convention that will help you identify it, such as “Image-web-lg.jpg” or whatever seems logical to you.
6. If you’re using an online service that automatically generates smaller file versions, you’re done. Upload your photo. If you’re making your own thumbnails, move on to step 7.
7. Resize the image once more, this time to fit within 240 x 240 pixels. You don’t need to sharpen again, as this image will be too small to make it necessary. Repeat step 5, saving the file in your thumbnails folder.