Your best bet is to pipe the output of ls to the foreach-object commandlet like this (% is an alias for foreach-object):
ls | %{rconvert $_ -resize 128x128}
Edit: ls outputs a list of FileInfo or DirectoryInfo objects onto the pipeline. To filter out the DirectoryInfo objects, use the where-object filter (? is alias):
ls | ?{-not $_.PSIsContainer} | %{rconvert $_ -resize 128x128}
If you need to access the full path name in your command, use the FullName property of the FileInfo object ($_ by itself will be resolved to $_.Name which is just the filename):
ls | ?{-not $_.PSIsContainer} | %{rconvert $_.FullName -resize 128x128}