Adding images to your work can help add to the value it, however it cannot be done at the expense of someone else's creative creation and livelihood. Many creators do however make their work free to use under certain circumstances, and many of those make it easy for you to find such works by applying a creative commons license to it.
For more about creative commons click here.
The creative commons website now comes with a search facility, and link to a beta version of an image search facility.
First lets look at the CC search page, by default Open Clip Art Library is selected, but you can select others on the page as well. Underneath the search box there are two tick boxes, by having these ticked your are applying the MOST restrictive filters. If you are not planning on charging for your work then you can take the 'commercial purposes' tick off, and if you don't plan on making any changes to the image, take the modify tick off. Then simply put in a key word to describe the image you are looking for.
In this example I searched for 'heart' without the two ticks. Once you find something you like click on the title and then you have a number of options for downloading, other associated images, and embed code.
There is also the Beta CC Image Search
Similar to the previous search, this will search across photographs and cultural works that you can use and remix, this time however the two filters for commercial use, and permission to modify are not ticked. If you are looking for clinical anatomy drawings or similar you are less likely to find them here.
If you have any further questions regarding the using of images or any other third party material in your own work please sign in and comment below or submit your request on QUEST www.quest.scot.nhs.uk