Version 1.0 of PixelCoaster has a problem with images that have a DPI (dots-per-inch) setting different than the computer screen's DPI (alternately known as ppi or pixels-per-inch). Either use images without an inherent DPI or use another image editing tool to convert the DPI setting to the default for your display, usually 72.
For example, you can use the Picture/Resolution menu item of GraphicConverter to adjust the DPI to 72. Uncheck the "convert picture" checkbox before pressing "OK".
With Version 1.0 of PixelCoaster you need to manually add a .tiff or .tif prefix to the filename you enter into the Save dialog box. Then it will automatically launch whatever application is designated as the default for TIFF image files, typically Preview.