Krita can be used to create .png images and it can crop images 
In the Brush Editor, under Brushtip - Predifined, you can use the +Clipboard button to make a brushtip from the clipboard.
Also, you can have more that one document open at a time and pump images from one document to another (using copy/paste) without using intermediate image file storage. Useful for when you’re in the early stages of developing brushtip images.
I suggest you try using Brush Mode - Gradient Map to get ‘speckled’ gold effects.
I played with that type of thing some time ago: Glittery!
You’d need to make a ‘gold’ coloured gradient, easy enough; then try some ‘speckled’ greyscale brushtip images to select the gold colours.