How can I optimize only certain thumbnail sizes?
WordPress usually has for each original image 3-5 thumbnails associated. Sometimes, after trying different themes and plugins the number of thumbnails that each image have associated can go to 10-20 or even 100! (yes, we've had customers that had that many associated for each image).
Sometimes it is useful to have all the thumbnails but many times some of these thumbnail sizes aren't needed anymore which means it doesn't make sense to have them optimized by ShortPixel.
Luckly it is easy to select the thumbnails sizes that you want ShortPixel Image Optimizer to compress: go to Settings/ShortPixel and then to Advanced tab. There, in the lower part you can exclude the thumbnail sizes that you don't want to have optimized: