For your website, not all images are the same. Here are some tips to make your images and your website look it's best.
1. Only use horizontal (landscape) orientation. Vertical images just don't look good online.
2. Make sure that all of your images are the same dimensions. Go with a standard size like 4x3, 5x7, or 9x6.
3. Optimize your images for web with a tool like http://webresizer.com/resizer/ - this will make your images load faster because they will be a smaller size - the smaller the better. You don't need print quality images for your blog to look amazing.