Marketing often consists of a number of different types such as paid for advertising or cost per click, on site marketing such as SEO and social media marketing. All of these different methods of marketing can work really well if they are designed in a way that is attractive to people and make them want to do what is being asked of them.
Often marketing and advertising on social media platforms works best when it is done alongside an image. The image may be an infographic explaining a process or showing a solution to a problem or it may be ad that is going to be shown in the paid for section. Regardless of what the reason for the image, it is important that you have good quality ones too. You may be tempted to try and create your own Images or banners to use along side them but that will probably not look very professional.
If you want decent images to use then you should ensure that you use a graphic designer who will be able to create a bank of images you can use across different social media platforms. This gives continuity to your brand and looks professional.