Strange...default opener for .PNGs _had been Firework 8. Something's happened where the system refuses to recognize Fireworks as an option.
I've stepped into Folder Options and attempted to set things up there. When I browse to 'Firework.exe' and click ok i just get dropped back at the list of programs with no change at all...no error...just refuses to recognize that i clicked Fireworks. It let me add a different program...just not FIreworks.
I've deleted the .png type altogether and tried to reassign...no luck. I'm able to assign .GIFs to Fireworks but not PNG.
At file types i'm expanding the 'System file types', right? All image types are opening in Opus's viewer on a double click even though 'Edit with Fireworks' shows as default.
What a mess...i'm not sure where to start describing stuff....
The thumbnails in a lister will show the Fireworks icon in the lower right for gif and jpgs...the viewer icon for pngs. Right clicking will present 'Edit with Fireworks' as a context menu choice (and that works). But insists that default is 'View in Directory Opus' - even tho Control Panel | Folder Options | Filetypes will show default is 'Edit with Fireworks'.
And for any image type Opus will steadfastely refuse to display Fireworks (a in the 'Recommended Programs' section of 'Settings | Filetypes | SYs Files |change. I can add it; set as default...choose ok then check and FIreworks is gone. As a test i tried adding acrobat as default...that stuck.
I've reinstalled fireworks and still have the same behavior. Grrrrrrr
Oh, I thought you'd got Fireworks working for GIF files. In that case I suspect you just need to go to Preferences / Double-Click / Files and either turn off Use internal picture viewer for... or change it to unregistered picture file types only.
When you first installed Opus the wizard will have asked if you want Opus to handle double-clicks on picture files and that's what initially determines this setting.
Edit: Just remembered it is also covered by this FAQ.