You can import PNG images or GIFs as screens into a project from Zeplin's Windows and Mac apps. To do this, go into the project that you want to import the images and select the "Edit > Import PNGs…" option from the desktop menu up top.
You can select any PNG image or GIF and click the “Import" button from the revealed menu. Selected PNGs will be uploaded to your project as images without any layers details.
☝️ Since this is an image not exported via the plugin, it will not contain any technical resources like layers, assets, and code snippets for screens imported as PNGs. To export the screens with their technical resources, you need to export them directly from the design tools through the plugins.