Simple Themes - Recommended Image Sizes
Featured image size
For your featured images, we recommend your images be at least 800x1100px, but no larger than 1700x2400px. This size will ensure your images display correctly in all areas of the theme (front page, category pages, etc).
Image sizes inside blog posts
The width of the blog post column is 700px. For the clearest possible display, we recommend uploading images roughly twice that size (1400px is great).
If page speed is a concern, then you can upload images right at 700px. Larger images really won't affect your page speed all that much, unless you have a huge number of images in one post.
If you're using the full width post layout, the post column is 950px and images should be at least that size.
Blog post slider
If you are using one of the Post Slider homepage options for your theme (example), your featured images will need to be a bit larger.
- Content width slider: 1060x1100px minimum featured image size
- Full width slider: 1600x1100px minimum featured image size