How i can convert my first picture to png format for preview?

How i can convert my first picture to png format for preview?

How i can convert my first picture to png format for preview? could you help me with it?
If i correct understand it’s work how on picture. But i saw functions
//retrieveInstance - 403 error
//retrieveInstanceFramesThumbnail - 403 error
//retrieveInstanceRendered - 403 error
I tried connect with your dicom pictures and have 403 error, Explain me please correct approach for show preview first frame.
Could you explain me please, how you did it preview? and important question you take preview from backend or it’s first frame from your dicom? or may be you use retrieveInstanceRendered function?
Thanks
describe this