Whilst content is important for SEO and for people who take an interest in your site, the images as probably going to be the first thing someone will notice. They will click onto your site and immediately be presented with the visual aspects of it. Your imagery has to be appropriate and sum up what you do accurately.
Choosing appropriate imagery may not be the cheapest option, but it is so important for your website. Use stock imagery if you have a subscription – you might be able to find some images with free usage rights but this will be a lot harder. Decide on the kind of appearance you are going for and choose images to fit in with this.
Even better, use your own photos. Work with a photographer to take photographs that represent your business, and that you will hold the copyright to. This gives clients a valuable insight into your business.