The - excludes the next term from results, so you are searching for nothing here.

A normal Google search returns no result, so Images must be handling it differently.

The dash is the NOT operator. This is the error that occurs when the query contains only NOT terms. It’s common to check for this special case in query processing and throw an error. Otherwise it’s easy to write a query that returns all or nearly all of the documents in the index.

Try `-1 2`, `-1 -2`, and `1 2`

More interestingly perhaps searching for “<term> -n to n” where n is an integer returns no results (e.g https://www.google.co.uk/search?q=google+-1+to+1

It's part of the search commands i.e http://lmgtfy.com/?q=peanut+-butter

searches for articles that contain peanut but not butter

unless... https://www.google.com/search?tbm=isch&sa=1&ei=HWWCXNPmLKeLj...

Doesn't work with regular google searches either