From ModMyiFone - Wiki
This tutorial will teach you how to fix those .png images from the iPhone which are in a format your Mac can't read. For more detailed info on the format of those .png images, check here.
Getting Started
I am assuming you already have some stock images you want to browse. If not, we have a guide here on how to get them.
The only other thing you need for this guide is FixPngDroplet, by chromaticrain.
Step 1. Installation
All right. When you downloaded FixPngDroplet, you got a file titled FixPngDroplet.zip on your Desktop. Go ahead and double-click it to un-zip it.
Inside you will find a file called libjpeg-libpng(universal).dmg.
Double-click it to mount it, and you will see a window pop up with libpng and libjpg - universal.mpkg. Double-click it to launch the installer.
Then follow through the installation steps.
Step 2. FixPngDroplet
Drag the FixPngDroplet.app from the FixPngDroplet folder into your apps folder, or wherever you'd like.
Step 3. The Conversion
Now grab whichever .png file you'd like to convert, and drag it directly onto the FixPngDroplet.app icon.
Assuming all went well, you should get a message which tells you the output message, which just means your original file should now be editable and viewable from your computer or any app you'd normally use to edit/view .png files!