XnView Software

I tried to convert jpg's to 31,25% of their original size, but the program does only use the numbers in front of the , and so the result was a convertion to 31%.

Is it possible to use float number %??????

In XnView 1.66, decimal values like 31.25% are not supported for resizing. If there's a real need for this, Pierre might add this.
(Please note that in English it's a decimal point '.', whereas in German it's a decimal comma ',').