One of the most difficult tasks in creating a research poster is finding image resources to use in your design. Not only do you need to find images that are relevant and have higher resolutions, but the images need to be copyright free, Here are a few online resources to find relevant, high-quality, and both copyright free and creative commons images:
Pixabay is a huge collection of stock images for any purpose. You can find stock photographs, illustrations, vector graphics, and even video on the site, download a high-quality, high-resolution version of that image, and place it in your design.
A project of the Wikipedia Foundation, Wikimedia Commons is an online repository of over a million free-use images.
The Noun Project is a collection of over 2 million icons that apply to just about anything. This is a very helpful resource if you're looking to use simple icons in your designs.
Pexels is very similar to Pixabay. Excellent source of high-quality images.