xampp/php/logs/php_error_log
Just delete php_error_log - try a couple of uploads and then see what it says in the log.
Your apache and MySQL logs are available through the XAMPP control panel - But you already knew that.
Just checking!
Edit:
I just installed XAMPP-Lite and a fresh download of Icy Phoenix on USB and I can't find any problem with uploading gif's or zip's (Didn't try any others).
Maybe yours has gone wonky, like I had to re-install XAMPP a couple of times before.
If you can't find/fix it - Try backing up your Db's and do a new install of xampp - because NOTHING my friend is perfect or runs for ever.