If you export the Annotations Measurements, see exporting measurements here, it is probably easiest to create your summary measurements (and standard deviations etc) in other software like Excel. If you script it (bottom of the help page) you could potentially create a CSV per image. There are also other forum posts on exporting measurements in various ways.
If you really want to dig into the coding, there are ways to create summary measurements, but they also would involve creating another annotation to store that measurement in. I have not looked into how to store measurements as part of the image, but it might be possible (for Export type, Image).
Make sure to check if any scripts you want to adjust are based on the same version of QuPath you are using.