You'll need to do text and image research at the same time, because background research helps you know what images to look for. Try making a mind map to plot your topic, the subtopics, the different perspectives/stakeholders, the existing problems that your designs could solve. Then, consider where information about each branch of your mind map would live. Once you know where that information typically lives, you can get started with research.
A mind map about sneaker design might include a branch about existing sneaker companies. Where does that information live? On company websites (Nike, Adidas, etc.), or in industry reports (search the web for _______ industry report, for example sneaker industry report). You might also have a branch about public opinion, so you could search social media sites for people's opinions about certain sneaker designs. You might have a branch of your mind map about sneaker history so you can come grab a sneaker book from the library.
Check out this book chapter for more mind mapping tips for designers.
Use the mind map tool below to get started! Search a broad subject (don't get too specific at first):
You don't have the luxury of doing all your research with one type of source, like how other majors/disciplines just use scholarly materials. As a designer you need to brainstorm all the types of information you might need and where that information typically lives. For example: