When someone sees an image of you or your art online, it needs to look great. Images that don't do your art justice will not entice a prospective gallery or collector to stick around and learn more.
Whether it's your own website, Facebook, Pinterest, or any online website, it's critical that your images look good and maximize your protection against theft.
I'm assuming here that you are getting professional-quality, high-resolution images of your work as soon as the piece is finished. You'll be using those images for your portfolio, to document your work, for filing copyrights, for printed materials, and much more. Hire a professional photographer if you can, and if you DIY, learn how to get great results. Do not use a cell phone, it's like committing professional suicide. Here's a helpful article on photographing artwork.
That done, you'll need to convert your high-resolution images to a web-friendly format like .gif or .jpg. That can be done in any number of image editing software including Photoshop (my fav but expensive, big learning curve), Photoshop Elements (powerful, best choice for most artists, easier to learn), iPhoto (on your MAC, try it).
JPG or GIF? GIFs can display up to 256 colors and are great for flatter-looking images like logos, drawings and cartoons. JPGs can display millions of colors and are better for detailed images and photographs, but file sizes are larger.
In addition to type of image file, you'll need to consider the final file size as well. Image and file sizes for use on the web are much smaller than for print. You don't want to upload a huge, high-resolution image. It will slow down the viewer's browser (and they'll get bored and click away) and it gives thieves a high quality image they can work with. You want to upload images that look great but don't print well.
Learn to use your image editing software for best results. After some trial and error you'll end up with a crisp, clear image that's appropriately-sized with as small a file size as possible.
For example, a really large jpg image with a lot of detail might be 1000 pixels on the longest side with a file size of under 200 kb. A smaller image that you use on a web page of you, your shows and events might be only 300 pixels wide with a file size under 50 kb.
Use the links above for more detailed info.
If you need help sorting it all out you can cut your learning curve waaayyyy down by having me tutor you one-to-one. Want a free 15-minute conversation about it? Sign up here.
As always, all my best to you and yours!