If you’re wanting to send pictures to someone, or plan on hosting them on a website, size can be rather important. I’m not talking about the dimensions of the image, but rather the file size. If you’re wanting to send a set of wedding photos to your aunt, she’s not going to want to download a bunch of 3MB pictures. Thus, you might want to try compressing the images. The trouble is that depending on the compression tool you use, you can lose a lot of quality.
If you’re looking for a quick and easy way to compress images, you might look into JPEGmini. Aimed primarily at high-resolution photos, this free web app claims to reduce your images to nearly a sixth of their original size. I’ve tried uploading a few myself, and have been rather pleased with the results. I still prefer to have something on my desktop like IrfanView, or even Photoshop to handle my resizing, but if you need to do this in a pinch, you won’t have to worry about installing software on your PC.