This is likely to happen if you do not regenerate thumbnails for images on your blog.
Here’s the issue: with your old theme, your blog feed and post pages were set to a particular width. Chances are, that width is not the same as your new <em>press theme. So, images need to be resized to fit the new theme.
Luckily, we’ve included an easy way for you to do this.
To resize all images on your site to fit within your new <em>press theme:
1. Go to Appearance > Customize > Welcome to [theme name]!
2. Click on the blue Regenerate Thumbnails button.
3. A new screen will load with information on how regenerating thumbnails works. Click on the grey Regenerate Thumbnails button.
4. A progress bar will load showing you what percentage of images have been resized. Keep in mind that if your blog has years worth of images, this may take a few minutes to complete.
5. Once the process completes, the images on your blog should all be the proper size! That’s it! It’s back to blogging business as usual.
Still having issues?
Occasionally, it’s possible that previous images you have been uploading to your site have dimensions that are so small, they will not look good with the new theme. In these cases, they may not be resized, because if their proportions are not large enough (especially for Retina displays), the quality will look so poor that the thumbnail regenerator will skip over them. For images that fall into this category, we recommend re-uploading a new, larger image in their place.