I reported it earlier here:
http://www.concrete5.org/community/forums/customizing_c5/cant-uploa...

I gave up and decided to just do a fresh install and upload images before I tried anything else.

So, I destroyed the old database completely, threw away the old files and then created a new database, dropped a fresh copy of concrete5.6.3 in the htdocs folder of MAMP. Installed it as usual. No errors were reported with the fresh install. I left the default template as is and went directly to the homepage and threw it into edit mode, added an image block and uploaded 3 small jpegs using the "upload multiple" button. This caused a very long spinning icon, but i waited until it finally finished. Then, a message of 'complete' came on the screen with the green button saying "continue". This was a lie, however, because it only uploaded 1 of the 3 images and reported the same error again:

Fatal error: Maximum execution time of 30 seconds exceeded in /Applications/MAMP/htdocs/Concrete5Tiopepe/concrete/libraries/3rdparty/Zend/Locale/Format.php on line 478

Attached is a also screenshot of the error in the File Manager for context.