When using wildcards outside of renaming you can use ? to match any single character.
You can't use ? when renaming, though. It is not supported as a find/replace pattern for wildcards. Only * is, to keep things simple.
Even if ? was supported when renaming, if you're matching *.png and replacing it with ??_??.png: You've got one * pattern in the old name and four ? patterns in the new name; the patterns being found don't correspond to the patterns being replaced.
If it was supported the syntax would probably be like this:
Old name: ????.png
New name ??_??.png
Type: standard rename (i.e. wildcards)
Since the wildcard syntax doesn't provide a way to specify which matched pattern is to be inserted in the new name, allowing ? probably wouldn't be a good idea. Wildcards are only suited to very simple find and replace patterns.
You can do what you were trying to do using regular expressions:
Old name: b(..).png[/b]
New name: \1_\2.png
Type: regular expressions