As cameras get more impressive, the physical size of photos gets larger and larger.

And when we upload them to the web, they take longer and longer to load on the page. And use up precious bandwidth.

So most sites that accept image uploads, often ask you to keep picture size down, in width, height and filesize.

The reasons to restrict width and height, are so that it doesn’t mess up the layout of the website.
Width and height are typically measured in pixels, for example 400px by 300px.

The reason to restrict filesize is to improve speed of page loading, and to restrict bandwidth.
When we build websites, we try to keep images below 35kb (kilobytes).

And most of us have bandwidth allocations from our ISPs, which can get used up quickly on graphics-intensive websites.

Two excellent free tools for resizing the height, width and filesize of your images are:
ShrinkPictures.com
Resizr

In both cases you can simply visit the site, upload your photo, specify the size or dimensions, and it will resize it for you.

If you want to work offline, you can use the common Microsoft Paint program that comes with your PC, under Accessories.  To resize, select Image from the top menu, then Resize/Skew. Enter say 80% and 80% in the horizontal and vertical boxes, and it will shrink your picture accordingly. I also use Paint for taking screenshots.  Hit the Print Screen button on your PC, to take a copy of your current screen, then Paste it into Microsoft Paint. Use the dotted-line-box to select the part of your picture you want to save, and select Crop, to get rid of the rest. Resize if necessary, and save as JPG.

For advanced resizing,  cropping, sharpening, balancing and retouching, I use Adobe Photoshop. But GIMP is a popular free open source alternative.

Have fun