Creative blocks are extremely frustrating. They often come when you're hoping to produce something awesome but can't manage to find the necessary inspiration to do so. When you get stuck, image searches can come to the rescue and save you from creative stagnation — you just need to take the right approach.
Google Image Search is one of the most helpful tools you can use for inspiration because you are, essentially, drawing upon the collective visual creativity of the world. But if you're trying to come up with a great idea for a poster design project, you don't want to just search for "cool poster design". While that will actually yield quite a few awesome posters, the results will be very unspecific.
Getting too specific is also a bad idea. Let's say you're designing a poster for a food market and so you search for "cool poster design food market" You'll end up with varied results, many of which will not be posters, and most of the posters won't look so great (e.g this one).
So how do you search? Like a thesaurus. Start by choosing single words you want your design to represent. Sticking with our example, fresh, green and food come to mind. Put those words into an image search and see how others have represented them. Some words will pull up beautiful images in the immediate results (e.g. fresh and green). Others may require you to dig down into the results a little more (e.g. food). Either way, your goal is to pick out a nice image that represents each word and put together a collection like the one to the right.
Once you do that, you've got visual elements you can draw upon when considering your design. The "food" image has an implication of love, the "fresh" image uses a splash of water, and the "green" image contains leaves. This not only gives you nice images to reference in your design, but tells you that a splash of water, leaves and love are all visual synonyms for the design you want to create.
When you're coming up with an effective visual design that communicates a message you don't want to give yourself the daunting task of reinventing the wheel. You also don't want to copy an existing style too closely because you'll then be circumventing the majority of the creative part of the task. Learning how people communicate core concepts visually, however, is a great way to learn how to express yourself in imagery in ways people already understand without losing your creative edge. Searching for images like you're searching a thesaurus can help you do just that.