You can visualize images directly from the "File Transfer" tab. Right-click on the image you want to visualize and select "View image" as illustrated below.
Please be patient, the image may take a while to be downloaded and parsed before being printed on your screen.
The visualization tool is based on the AMI Medical Imaging (AMI) JS ToolKit (https://github.com/FNNDSC/ami). It supports multiple file formats, among which Dicom (.dcm), Nifti (.nii, .nii.gz) and MHD/(Z)RAW files.