If so, where this file should be
placed?
Like Richard said, it's read automatically by GNUstep. Just add it to the PhotoClip_RESOURCE_FILES of your make file.
> You probably also want to create image files for the different types
The PhotoClip.tiff icon represents the program itself. For the file
types I'd use something like ps.tiff that comes with WindowMaker
(although I haven't seen jpeg.tiff or png.tiff ...). Ideally we want
thumbnail preview.
That's a good point! GWorkspace does create thumbnails of images, but GNUstep doesn't handle all images (as I recently discovered, 48-bit images aren't officially supported).