how to open a Dataset of 2D xrays in PNG format ?

hello , i have a dataset of spine xrays (2D) in PNG format , i want to annotate them using 3d slicer and monai label , but i cant load the image in
i found that converting tthe dataset to nifti .nii might be a solution but i don’t know how to do this .
please i hit a blocker i really need help!