There are two options at the moment. I have not personally tried PICT files with either, so this is just based on what they say they can do and not experience of PICT files definitely working in them or working well:
-
Quick View Plus (Opus forum thread) says it will handle some PICT files here. ("Bitmap only", whatever that implies. I don't know much about the format.) It is not free, but once installed it should work in the Opus viewer pane, as well as the File Explorer viewer pane and in Outlook.
-
The Opus-specific Universal Graphics Viewer plugin has Macintosh Quickdraw/Pict: .pic .pict .pict2 .pct in its list of supported formats, and is free. That's probably your best bet if you want something quick to sort through the files. Note that that plugin causes some problems with some formats so you may not want to leave it enabled when not using it for specific things. Trying now, it also looks like the author's website has been suspended. I don't know if that is just temporary, but it makes getting the plugin rather difficult.
There's also the third option of writing a viewer plugin to handle the format, if you are a programmer. Writing simple bitmap viewer plugins for Opus is very simple, in terms of the Opus specific parts, as they just need to pass a bitmap to Opus and it then does the rest. If there is an existing library to turn PICT files into a bitmap then it should be fairly straightforward. I'd be happy to help, but couldn't do the whole thing myself due to other demands on time at the moment.