It sound like Impression cannot find your original PNG files. You need to add the path to the folder containing your PNG's to Impression (Edit>Options>Paths).
What I do is to have a single Impression file (Trees.irf) which contains a single instance of each of my tree blocks. When working on a new file, you can open another IRF file as a Library (you can do this for fills as well). To do this open the Blocks Palette (F4), and click the FileOpen button top left in the palette. In my case I open my Trees.irf, which then appears in the list of block collections. Click on it and you see all the tree blocks it contains. Right click those you want in the current drawing and click 'Add to Saved'. You know have them available for use.
Hope this helps...