Hi David, great questions!
1. Most JPG modifications, but specifically image resizing, is a lossy operation. The JPG quality determines how much loss is acceptable. More on JPG quality on our EWWW IO docs site: https://docs.ewww.io/article/12-jpg-quality-and-wordpress
2. I’m not certain what you’re asking. If you’re wondering whether other plugins can alter the quality, there are lots of plugins can change the JPG quality used by WordPress. See, the actual resizing done by Imsanity is performed using core WP functions. Thus, any plugin that alters the JPG quality might override what is set by Imsanity.
If you’re asking whether the quality can be changed without resizing an image, that’s not possible with Imsanity.
If you’d like to actually optimize the compression of an image further, I recommend an image optimizer plugin like our EWWW Image Optimizer.
As for other formats JPEG=JPG, but PNG image resizing doesn’t have a quality level, it has a compression level. Since PNG is a lossless format (normally), the only thing that can be changed is how much “effort” is put into making the PNG encoding as efficient as possible. That’s not configurable by Imsanity and as far as I know, the core image resizing functions do not change the default compression parameters for PNG images.
