We are getting many complaints from customers that the captcha image is unreadable. Also there is no way to get a new captcha image without refreshing the whole page and losing all the other entered information.

Is there any way we can adjust the parameters on the captcha, and any way to add a 'get new captcha' button?

Spot,

Currently no, but PrintShop Mail Web 7.1 will have new Captcha features for these reasons. You'll be able to refresh the captcha image, or disable the feature altogether if necessary.

There's no exact ETA for PSM7.1, but if all goes as planned it should be around January 2011.