Every 17th Avenue theme calls for new image sizes. If you had existing images on your site when you installed the theme, your images don't yet know that they are supposed to be using those new image sizes. You'll need to resize your images using a plugin called Regenerate Thumbnails so they can fit properly in your new theme.
- Install and activate the Regenerate Thumbnails plugin.
- Once installed, go to Tools > Regen. Thumbnails.
- Click "Regenerate All Thumbnails."
- If you have a lot of images on your site, this may take a while to complete. Do not exit the page until the process has 100% completed.