From a multi-megapixel camera or even a smartphone, photo files can be
You may think that these days the file size of photos is not important. Storage capacity grows ever vaster. Processing gets faster.
You have only to try emailing a few multi-megapixel photos to realise that this is not necessarily true. Size does still matter.
Photoshop or any number of photo editing apps will downsize photos to email, post in a gallery or show on a web page. It's not too much trouble - but do you really need to do the resizing yourself? Easier to let mail apps, galleries and browsers size your photos automatically.
Beware though. You could be wasting time and money.
Your picture may be bound for a screen too small to display it in all its glory. A receiving gallery may downsize it automatically, but only after you've sent it at full size.
Not only that. Behind the scenes, a gallery like Flickr could be creating and storing several further versions at different sizes. The size you sent will be the biggest, so it looks like a good idea not to make this too enormous.
In any case, huge images don't always fit to screen and may be inconvenient to view on many monitors.
What happens if you load an overweight image to a blog or web page? The browser not only has to load the larger file. It has to take the time to resize the image before it can display it. This slo-o-o-ows things down, especially if the viewer does not have a fast connection.
Moral: Determine the maximum size your picture will be shown at. Reduce your big multi-megapixel photo to that size yourself before you send it out into the world.
When downsizing photos, make sure you are reducing a copy. Otherwise you might lose the original.
Remember that jpg image files, the form in which many camera phones save images, are lossy. That is, every time you re-save a .jpeg, it loses definition. After too many times, your .jpeg starts to show weird artefacts. Best to convert your photo to a more robust form like a Photoshop .psd or a .tiff file before you start.