A lot of people don't realize the importance of using the right image file size. They upload a photo they took with their digital camera and usually pay no attention to the size of the image. Sometimes these image files are in MB rather than KB and take forever to upload.
Guess what? They make your site load slower than pouring molasses in a Minnesota winter. As if that's not enough of a downside, consider this: Google ranks slow loading sites low, low, low. To paraphrase Martha Stewart in the opposite: "That's a bad thing."
The key is to always make your image size in pixels per inch (ppi) with 72 ppi being the web protocol that ensures fast load time and good resolution.
Just about everyone has some kind of image software with Adobe PhotoShop being the gold standard with an attached price tag that reflects this.
If you don't want to pay the big bucks for that then use any of the free image manipulation softwares that can be found by doing a Google serach for that keyword phrase.
You want to do everything you can to keep people on your site once they've clicked its URL. Nothing makes people leave faster than a site that takes too long to load.