The photos have been added to portfolio pages by means of a featured image. We made this choice, because we prefer this order at the team page (first photo, then name [title of portfolio page] and then function).
However, we have some troubles with the display of the photos at the team page on some screens. They show blurry. It seems to be related to the size of the screen and the resolution. Therefore, some screens display the photos well and others do not.
We already tried to upload photos with larger sizes and higher resolution. In addition, we looked at the media settings and tried some adjustments. However, both did not deliver the desired result.
The theme actually adds a “Recommended image sizes” info into Settings → Media. But surely, if you made your theme’s content area wider, set the images to your preferred sizes.
However, the size you set is applied for newly uploaded images in WordPress. To refresh already uploaded images to new sizes you need to use a plugin such as Force Regenerate Thumbnails. For more info please refer to theme documentation.