So i just rename the miscdata[80].brres to miscdata[80].png?
No you add .png at the end of the file inside the MiscData[80]; for example, MenSelchrPrevbase.15.png. I'm pretty sure that's not a real file found in a selmap but I don't remember any of the actual names so I just gave a rough example.
Oh wait, if you're saying when you export your .brres file to your desktop and you can't preview it or anything, that's normal; only BrawlBox can read those files.