Getting a "500 Internal Server Error" when uploading an image asset
ok i solved the problem by following the answer to this question here:

FastCGI error when accessing a local development site using MAMP Pro

even though the ajax call didn't generate an apache error at the moment it was called, it must have been something to do with fastcgi in general, cos the answer to that question worked for me! 

i have never had this problem before, and i have heaps of other local sites which also do file/image uploading. so strange. and uploading of other file types (eg pdf) into the assets area worked no problem! doesn't make sense. but as a developer, sometimes you just accept the mystery of it all...

UPDATE

the problem started rearing it's ugly head again, even though i hadn't touched the httpd.conf edit i did to fix it. so i just deactivated the fastcgi module on the MAMP Pro dashboard. et voila, we're back in business....