Why am I getting a warning about a conflict with a thumbnail regeneration plugin?

ShortPixel Image Optimizer optimizes images, and almost all the plugins that are used to regenerate thumbnails (or have this option available) can make ShortPixel "forget" it has optimized your images and trigger a re-compression. This action causes your images to be overoptimized. Here's more information about this particular problem: Why are my images re-compressed?

Some plugins that can cause the over-optimization are:

• Simple Image Sizes
• WooCommerce - has a thumbnail regeneration component
• Regenerate Thumbnails
• Regenerate thumbnails and delete unused

The list is not limited to those listed. As mentioned, almost all thumbnail regeneration plugins can conflict. 

There are two possible solutions for this:

1. Use reGenerate Thumbnails Advanced which is compatible with ShortPixel Image Optimizer.
2. You can deactivate ShortPixel Image Optimizer while you perform the thumbnail regeneration, and once you finish, re-activate it. This will not prevent losing ShortPixel's metadata (which means that ShortPixel won't remember whether an image was optimized), but at least it won't automatically re-compress the images and lead to over-optimization.