When an image is uploaded to Photos app, several reduced versions (thumbnails) of this image are automatically created. Thumbnails of different sizes are displayed in the app's backend and on your website, i.e. in an online photo gallery.
The number of created thumbnails and their sizes depend on the app settings.
Default and custom thumbnail sizes
In the Settings screen, several default sizes are always available.
These default sizes are used in the backend and can also be used on the website. Which of them are actually used in the public photo gallery depends on your design theme. To find out which thumbnail sizes are required by the design theme, proceed to "Design" screen, select the theme from a drop-down list and scroll down to the thumbnails description paragraph in "Appearance" section.
If this list contains thumbnail sizes different from default ones displayed on the settings page then you might need to set up custom sizes as described below.
When you need to set up custom thumbnail sizes
It is recommended to set up custom sizes if they are listed on the design theme settings page or in the theme user manual.
If you have applied a new design theme requiring non-standard thumbnail sizes after you have uploaded at least some photos to your account, then you need to ensure that those "old" photos are correctly displayed with new sizes applied. To do so:
- Set up new custom sizes using the description provided below.
- Enable setting "Allow on-demand generation of custom-sized thumbnails". With this option enabled, Photos app automatically creates the requested thumbnail once it is displayed on a website page. The image is displayed to a visitor and is cached for future use.
How to add custom thumbnail sizes
When adding custom sizes, choose one of the available image scaling and crop options using their descriptions below.
Fit into a square 
Original image is scaled down proportionally so that neither its width nor height exceeds the specified value.
Limited by width [200x0]
Original image is scaled down proportionally so that the resulting thumbnail width does not exceed the specified value.
Limited by height [0x200]
Original image is scaled down proportionally so that the resulting thumbnail height does not exceed the specified value.
Square crop [200x200]
A square of maximum size is cropped from the middle of original image and the result is scaled down to the specified size. If the original image has a square shape, it is simply scaled down.
Rectangular crop [200x100]
A rectangle of the maximum size, with dimension proportions equal to that of the original image, is cropped from the middle of the original image, and the result is scaled down proportionally to the specified values. if the original image has the same dimension proportions, it is simply scaled down.