I just tried a search using the words "field of red maize" on iStockphoto, and I don't receive any results. But using the words field and red maize will give me results. Why does the word "of" ruin everything (I've been wondering about this for a while). It's not a big deal to me; I'm just curious.

Did you put this search phrase in quotes? If so, it would only return files that contained exactly that phrase as a single keyphrase. And, given the disambiguation function and controlled vocabulary, a file with that as a key phrase would never make it past inspection.

I tried it with quotes and without. I was just wondering if it was a glitch or something because at other sites, it still brings up results.

I used to work in the search data team for one of the big full-price stock sites. On that site words like 'of', 'a', 'the', 'on' etc were added to what's called a stop-list - i.e. the search engine is told to ignore them.

In addition these words were never added as keywords in the controlled vocabulary (but could be added as parts of phrases acting as synonyms to preferred terms - so for instance 'bottle of' could be a synonym to 'bottle', so that a search on Bottle Of Milk would actually just be a search on Bottle AND Milk).

I notice now on the iStock site that if you search on Field Of Maize you get back the following prompt:

Field AND of AND Maize (Cereal Plant)

This indicates to me that 'of' has been added to the controlled vocabulary by someone who didn't think through the implications...

"red maize" searched using quotation marks has no results, i.e. no-one has actually keyworded the maize in their image specifically as 'red maize'.
red maize (no quotations) brings up any photos with both red and maize as keywords. These will not necessarily show red maize. And with spam still being rife, some have no red and/or no maize. No surprise there. 

Good points and all true regarding the original posters search problem, however Field Of Maize brings back 0 results too. The keywords Field and Maize are two terms you'd expect to find added together. The problem in that case is that Of has been added to the controlled vocabulary and the search is looking for images combining the three keywords Field, Of and Maize.

The solution would be to either delete the keyword Of, add it to a stop-list or make Field Of a synonym to the keyword Field. Probably best to do all three.