How can I optimize only certain thumbnail sizes?
By default, WordPress generates up to 7 thumbnails for each image you upload to your site. However, after trying different themes and plugins, the number of thumbnails that each image has associated can go to 10-20 or even 100 (yes, we've had customers that had that many)! Since ShortPixel Image Optimizer is spending 1 credit per image or thumbnail optimized, that means that every time you upload an image, you will spend lots of credits.
Usually, it is useful to have all the thumbnails optimized, but in some cases, some of these thumbnail sizes aren't needed anymore, so it doesn't make sense to have them optimized by ShortPixel and waste credits.
Luckily, it is easy to select the thumbnails sizes that you want ShortPixel Image Optimizer to compress: go to Settings > ShortPixel and then to the Advanced tab. There, on the lower part you can exclude the thumbnail sizes that you don't want to have optimized: