Hello guymartin2.
I understand your concerns with your Photosmart 7525. Your printer is compatible to use "Borderless Printing". I will provide a couple of steps to try and resolve this issue.
Please follow this entire HP document on How to Print Borderless Images.
Since you are printing photos from Picasa 3, I will provide a website that gives you step-by-step information on printing without borders here.
The reason why you are unable to print borderless using the ePrint App on your iPhone is because this program doesn't support this option, as you can only perform this on your computer.
I understand Picasa 3 is giving you an option for 4x6 photos, but it's displaying 4.2 x 6.2. You want to make sure you have the Photosmart 7525 full feature software installed all your computer. Also, there could be a problem with Picasa 3 if it's displaying those options, but I will provide some information on Using Picasa 3.
Let me know if your have any further questions.