since your suggestion didn't work (changed the respective WebPart property and still it was ignoring me) and since in the group edit page and maybe in other places too it was trying to use that same missing size, I created a script to generate all the missing sizes (I'm a bit naughty here in that I don't call some external icon resizer, but just copy the 640_t version over as 576_t one if it is missing - after all they were both JPGs arround 8K in size from what I have seen).
You can place the following batch file anywhere under your site and run it, it will search recursively all subfolders. Best is to place it under Upload folder of MonoX
::-- BATCH file that creates an *_576_5.* file from an *_640_t.* one (copying it)
::-- Author: George Birbilis (http://zoomicon.com)
::-- Credits: String replacement based on http://www.dostips.com/DtTipsStringManipulation.php
::-- Loop for all files recursively --::
FOR /R %%f in (*_640_t.*) DO CALL :process %%f
::-- Per-file actions --::
:: Display progress...
::ECHO Processing %*
<nul (set/p dummy=.)
IF NOT EXIST %toFile% CALL :generate %fromFile% %toFile%
::-- Generate missing file --::
ECHO COPY %*