Images are pretty critical when engaging with an audience. We all love pictures, graphics and 
carefully selected images. On websites, images fall into three types: decorative, illustrative and branding.
Your logo (Branding)
I recommend to do all things in setting up a business as cheaply as possible and getting the logo is the same, but you don’t want to go too cheap as your logo is critical to your brand.
The best thing I can recommend is to use a site such as 99designs. With these sites you can get a whole army of designers competing for the prize money that you lay down – £175. So you get a vast array of designs to choose from – it is really rather good value.
To get it any cheaper than this you’re just going to have to ask around people you know (if you can’t do it yourself), post on Facebook or Twitter and ask if someone can do you a favour. You’re pretty likely to know someone who knows someone who is adept at designing something.
Illustrative
Anything illustrative is to show off a product and what it can do. You might use these on ebay, if you’re selling an ebook or any other type of product online.
Good quality photos of your products will increase your customers’ perception of your professionalism, which means you will be able to increase your sales prices.
Decorative
You may want to decorate an article with such an image. You can get these photos from:
– Clipart.com
– Fotolia.com
– istockphoto.com
Which file format shall I use?
Jpeg or jpg is the best format to use, unless you need a transparent photo (no background) then use a png file. You only need to use gif if you are using a black and white photo.