When posting photos online, you do not need nearly as many pixels as you do for printing. This also goes for images that will only be viewed on-screen such as in a slideshow or presentation.
Having too many pixels makes it difficult to view photos on a monitor and it makes the file size much larger--something you need to avoid when posting photos on the Web or sending them by email. Remember, not everyone has a high-speed Internet connection or a large monitor, so sizing photos down before sharing them is the courteous thing to do. The recipient can always ask for a larger file if they wish to print it--this is always better then sending large files without asking first.
In most cases, you will want to do all three of these things.
Since PPI and DPI are only relevant to print size and quality, when dealing with digital photos for the Web, you need only look at pixel dimensions. Most 24-inch desktop monitors today have a resolution of 1920 by 1080 pixels, so your images need not be larger than this for on-screen viewing. Laptops and older computers will have an even lower screen resolution, so keep that in mind as well. The smaller the pixel dimensions of an image, the smaller the file size will be.
When sharing photos online, 1200 by 800 pixels is a good average size to go with.
If you wish to reduce file size even further, reduce your images down to 800 by 600 or even 640 by 480.
If you're sure your recipients or website visitors won't mind waiting a little longer, you may want to go as large as 2000 pixels wide, but anything larger than that is going to be too large to see in the majority of computer monitors without scrolling.
File compression is another way to make your photos smaller for online use. Most cameras and scanners save in the JPEG format and this format utilizes file compression to keep the file size down. Always use the JPEG format for photographic images you will be sharing online. It is a standard file format that any computer can read. JPEG compression can be applied at various levels, with image quality and file size having an inverse relationship. The higher the compression, the smaller the file, and the less quality it will have.
What Resolution Is Best for Photo Printing?